Package [com.unity.package-manager-ui@2.1.2] cannot be found

Context

this error is known to occur when you import the Prespective Rendering Pack into a version of Unity3D > 2019.4.x (e.g. 2020)

Resolution

1 | When you get the error when opening the project press continue to enter Unity3D:

Open

2 | In the Unity3D Editor go to Window > Package Manager

Open

3 | Find the offending package in the list of currently used packages

Open

4 | Press the ‘Update to X.X.X’ (in the image above 2.2.0) button in the lower right corner of the window

5 | The Issue should now be resolved