I am trying to open unity project in Unity3d 2018.3.9. then i am getting an error

An error occurred while resolving packages
com.unity.xr.arcore:Cannot connect to 35.238.21.138:443'(Error code : ECONNREFUSED)
com.unity.xr.arfoundation:Cannot connect to 35.238.21.138:443'(Error code : ECONNREFUSED)
com.unity.xr.arkit:Cannot connect to 35.238.21.138:443'(Error code : ECONNREFUSED)
com.unity.xr.openvr.standalone:Cannot connect to 35.238.21.138:443'(Error code : ECONNREFUSED)

Restarting Unity, Help > Reset Packages to defaults, manually adding the dependencies to the manifest.json doc, and reinstalling Unity did not fix this issue. Please resolve. Very frustrating.

Failed to resolve packages: The file [C:\Users\Computah\Desktop\Unity Projects\GameName\GameName\Packages\manifest.json] is not valid JSON:
Unexpected token '' at 1:1
{
^. No packages loaded.

A re-import of the project may be required to fix the issue or a manual modification of C:/Users/Computah/Desktop/Unity Projects/GameName/GameName/Packages/manifest.json file.

Hello, I Have 2018.3.7.f1,
and Both TextMeshPro and Packages Manager do not show in Window. I had a working project in Unity 2017
LTS, I then updated to this and I have this problem.

I tried:
-Help-->Reset Dependencies.
-deleting cache
-deleted old TextMeshPro from Asset
-restarting Unity

this is the manifest:
"dependencies": {
"com.unity.ads": "2.0.8",
"com.unity.analytics": "3.2.2",
"com.unity.collab-proxy": "1.2.15",
"com.unity.package-manager-ui": "2.0.3",
"com.unity.purchasing": "2.0.3",
"com.unity.textmeshpro": "1.3.0",
"com.unity.modules.ai": "1.0.0",
"com.unity.modules.animation": "1.0.0",
"com.unity.modules.assetbundle": "1.0.0",
"com.unity.modules.audio": "1.0.0",
"com.unity.modules.cloth": "1.0.0",
"com.unity.modules.director": "1.0.0",
"com.unity.modules.imageconversion": "1.0.0",
"com.unity.modules.imgui": "1.0.0",
"com.unity.modules.jsonserialize": "1.0.0",
"com.unity.modules.particlesystem": "1.0.0",
"com.unity.modules.physics": "1.0.0",
"com.unity.modules.physics2d": "1.0.0",
"com.unity.modules.screencapture": "1.0.0",
"com.unity.modules.terrain": "1.0.0",
"com.unity.modules.terrainphysics": "1.0.0",
"com.unity.modules.tilemap": "1.0.0",
"com.unity.modules.ui": "1.0.0",
"com.unity.modules.uielements": "1.0.0",
"com.unity.modules.umbra": "1.0.0",
"com.unity.modules.unityanalytics": "1.0.0",
"com.unity.modules.unitywebrequest": "1.0.0",
"com.unity.modules.unitywebrequestassetbundle": "1.0.0",
"com.unity.modules.unitywebrequestaudio": "1.0.0",
"com.unity.modules.unitywebrequesttexture": "1.0.0",
"com.unity.modules.unitywebrequestwww": "1.0.0",
"com.unity.modules.vehicles": "1.0.0",
"com.unity.modules.video": "1.0.0",
"com.unity.modules.vr": "1.0.0",
"com.unity.modules.wind": "1.0.0",
"com.unity.modules.xr": "1.0.0"
}
}

PLEASE HELP ME IT'S 5 DAYS I AM Struggling with this !

I to am having this issue. I created a new project in 2018.3.4f1 on one machine. (Which backs up to Google Backup & Sync.)
I'm now using a different machine which has just had unity updated to the same version. I've opened my project (which complained was created in a different version of unity?) and the package manager is empty, and I have 8 errors which I can't clear. (relating to the LWRP (which I've stopped using) and the post processing stack.)
I can't install the missing packages as the window is empty.
Clicking reset packages to default did not resolve the issue.
Restarting the editor did not resolve the issue.

Some of the errors are below:

"Library\PackageCache\com.unity.render-pipelines.lightweight@4.1.0-preview\Runtime\LightweightRenderPipeline.cs(9,29): error CS0234: The type or namespace name 'PostProcessing' does not exist in the namespace 'UnityEngine.Rendering' (are you missing an assembly reference?)"

"Library\PackageCache\com.unity.render-pipelines.lightweight@4.1.0-preview\Runtime\LightweightRenderPipelineCore.cs(3,29): error CS0234: The type or namespace name 'PostProcessing' does not exist in the namespace 'UnityEngine.Rendering' (are you missing an assembly reference?)"

"Assembly has reference to non-existent assembly 'Unity.Postprocessing.Runtime' (Packages/com.unity.render-pipelines.lightweight/Runtime/Unity.RenderPipelines.Lightweight.Runtime.asmdef)"

"Assembly has reference to non-existent assembly 'Unity.Postprocessing.Runtime' (Packages/com.unity.render-pipelines.lightweight/Editor/Unity.RenderPipelines.Lightweight.Editor.asmdef)"

And so on.

I encountered this issue today when updating an existing project from 2018.2 to 2018.3. I need the package manager so I can download the standalone OpenVR. But as the original issue says, there is no Package Manager in the Window dropdown. I tried Help -> Reset packages to defaults and accepted the warning that the project might be damaged, but that did not have any apparent effect. I have restarted Unity several times before and after doing that, but the Package Manager option has not appeared yet.

My issue perhaps was related to the manifest file in my project being incomplete.
I fixed the issue by updating MyProjectRoot/Packages/manifest.json with the following (you can pick and choose modules after in the package manager UI)

{
"dependencies": {
"com.unity.ads": "exclude",
"com.unity.analytics": "exclude",
"com.unity.standardevents": "exclude",
"com.unity.package-manager-ui": "1.9.11",
"com.unity.purchasing": "exclude",
"com.unity.modules.ai": "1.0.0",
"com.unity.modules.animation": "1.0.0",
"com.unity.modules.assetbundle": "1.0.0",
"com.unity.modules.audio": "1.0.0",
"com.unity.modules.cloth": "1.0.0",
"com.unity.modules.director": "1.0.0",
"com.unity.modules.imageconversion": "1.0.0",
"com.unity.modules.imgui": "1.0.0",
"com.unity.modules.jsonserialize": "1.0.0",
"com.unity.modules.particlesystem": "1.0.0",
"com.unity.modules.physics": "1.0.0",
"com.unity.modules.physics2d": "1.0.0",
"com.unity.modules.screencapture": "1.0.0",
"com.unity.modules.terrain": "1.0.0",
"com.unity.modules.terrainphysics": "1.0.0",
"com.unity.modules.tilemap": "1.0.0",
"com.unity.modules.ui": "1.0.0",
"com.unity.modules.uielements": "1.0.0",
"com.unity.modules.umbra": "1.0.0",
"com.unity.modules.unityanalytics": "1.0.0",
"com.unity.modules.unitywebrequest": "1.0.0",
"com.unity.modules.unitywebrequestassetbundle": "1.0.0",
"com.unity.modules.unitywebrequestaudio": "1.0.0",
"com.unity.modules.unitywebrequesttexture": "1.0.0",
"com.unity.modules.unitywebrequestwww": "1.0.0",
"com.unity.modules.vehicles": "1.0.0",
"com.unity.modules.video": "1.0.0",
"com.unity.modules.vr": "1.0.0",
"com.unity.modules.wind": "1.0.0",
"com.unity.modules.xr": "1.0.0"
}
}

The "com.unity.modules.*" elements were missing along with the "com.unity.package-manager-ui"

Hope this helps anyone who has the same issue.

Restarting unity doesn't work, the project code is riddled with dependancies to Animation, Audio and WWW. This is a big problem! Going to have to revert back to an older unity to try and sort this out.