I am trying to import the unity’s post processing stack to Unity 2021.1.6f1, but when I install it from teh package manager, I get this error message:
Assets\PostProcessing\Editor\PropertyDrawers\MinDrawer.cs(6,34): error CS0104: ‘MinAttribute’ is an ambiguous reference between ‘UnityEngine.PostProcessing.MinAttribute’ and ‘UnityEngine.MinAttribute’
Why do you have this in your Assets folder? If you only installed PP in the package manager, you wouldn’t have PostProcessing in your Assets folder. Make a backup of that folder and remove it from your project and see what happens.
Also make sure you install PostProcessing from the Unity Registry not from the Asset Store (My Assets).