If Visual Studio isn't recognizing certain types in your Unity project, it's likely due to a compatibility issue or misconfiguration. First, ensure both Unity and Visual Studio 2017 are compatible and properly set up. Try reimporting assets in Unity and rebuilding the solution in Visual Studio. Check scripting define symbols and references, and update extensions if applicable. If problems persist, seek help from Unity or Visual Studio communities. Restarting both programs may also help. With these steps, you should be able to troubleshoot and resolve the issue efficiently.

I Like this post thanks

Thank You So much this is very helpful information.

Experienced the issue in 2020.3.14 with Visual Studio 2019 and 2022
Issue no longer observed in 2021.3.10

please check this issue.
https://stackoverflow.com/questions/66019773/how-to-add-gradient-color-to-text-element-in-wix-site/66019799#66019799

Thanks for sharing this useful information - http://issuetracker.unity3d.com/issues/referenceoutputassembly-key-is-set-to-false-in-project-references

In Package Manager, find "Visual Studio Code Editor" and upgrade to version 1.2.3 to fix the issue

I have received an update from Unity that this issue has been resolved in `com.unity.ide.vscode@1.2.3`. You can see the changes to the code mirrored here: https://github.com/needle-mirror/com.unity.ide.vscode/commit/efa6bd0ed1ed0129f6f5c47c81f1d4b2bbd9ac53.

So update the package `com.unity.ide.vscode` to version 1.2.3 and everything should be working again.

Got this error on 2019.4.12f1.

Thank you @peabnuts123 for the workaround. I am new to issue tracker and not sure where to file this issue.