An error occurred while resolving packages: Project has invalid dependencies: com.unity.package-manager-ui: Package [com.unity.package-manager-ui@2.1.2] cannot be found Package com.unity.textmeshpro@2.0.0 has invalid dependencies: com.unity.ugui: Package [com.unity.ugui@1.0.0] cannot be found
Assembly has reference to non-existent assembly 'Unity.ugui' (Packages/com.unity.textmeshpro/Scripts/Editor/Unity.TextMeshPro.Editor.asmdef)解决：
Were you porting the project from an older Unity version? Anyway, the solution for me was to go to: C:\Users\username\Documents\MyProject\Packages (Or simply your project folder) and deleting manifiest.json. If it doesnt help, try pasting in between the lines of the manifiest.json file this line of code: "com.unity.package-manager-ui": "2.1.2",
把工程目录下的Packages目录中的manifest.json删掉，然后会自动重新生成就可以了
参考：
Package [com.unity.package-manager-ui@2.1.2] cannot be found
https://answers.unity.com/questions/1626849/package-manager-missing-unity-201910f2.html