How to reproduce it:
1. Open the user's attached project "WanderingStar.zip"
2. Build and Run it on Android
3. Observe the Console window

Expected results: Building for Android succeeds
Actual results: Building for Android fails. Exception appears in the Console window:
Exception: Failed running ...\2021.1\2021.1.6f1_c0fade0cc7e9\Editor\Data\il2cpp\build/deploy/netcoreapp3.1/UnityLinker.exe @"..../<Project name>/Temp/StagingArea/assets/bin/Data/Managed/response.rsp"
stdout:
Fatal error in Unity CIL Linker
Mono.Cecil.AssemblyResolutionException: Failed to resolve assembly: 'Unity.RuntimeSceneSerialization, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null'

Reproducible with: 2020.3.7f1, 2021.1.6f1
Could not test with: 2018.4.34f1, 2019.4.26f1 - UI Toolkit is not introduced yet, 2021.2.0a17 - due to the MARS error

Note:
- Issue not reproducible after removing a Unity MARS Companion Core package
- Reproducible with a new project, when it contains Unity MARS Companion Core and UI Toolkit packages
- After removing UI Toolkit package from the user's project, in 2021.2.0a17 MARS error remains: "Library/PackageCache/com.unity.mars@1.3.0/Runtime/Scripts/EditorComponents/SimulationRenderSettings.cs(86,34): error CS0266: Cannot implicitly convert type 'UnityEngine.Texture' to 'UnityEngine.Cubemap'. An explicit conversion exists (are you missing a cast?)"