UnityIAP? Assembly [...] will not be loaded due to errors:

I just added UnityIAP and got the following errors:

  Assembly 'Library/ScriptAssemblies/Assembly-CSharp-firstpass.dll' will not be loaded due to errors:
 Reference has errors 'Stores'.
 
 Assembly 'Library/ScriptAssemblies/Assembly-CSharp.dll' will not be loaded due to errors:
 Reference has errors 'Stores'.
 
 Assembly 'Library/ScriptAssemblies/Assembly-CSharp-Editor.dll' will not be loaded due to errors:
 Reference has errors 'Assembly-CSharp'.
 
 Assembly 'Assets/Plugins/UnityPurchasing/Bin/Editor.dll' will not be loaded due to errors:
 Reference has errors 'Stores'.
 
 Assembly 'Assets/Plugins/UnityPurchasing/Bin/Stores.dll' will not be loaded due to errors:
 Unable to resolve reference 'UnityEngine.UI'. Is the assembly missing or incompatible with the 
 current platform?
 Reference validation can be disabled in the Plugin Inspector.

Nothing is working anymore, no script is working because of this errors. I already tried to reimport the plugin in multiple ways.

Thanks in advance!