Multiple master files selected for load. Load operation aborted.

In the following file:
C:\Program Files (x86)\Steam\steamapps\common\Skyrim\SkyrimEditor.ini
C:\Program Files (x86)\Steam\steamapps\common\Skyrim Special Edition\CreationKit.ini

Add next:
bAllowMultipleMasterLoads=1

[General]
bAllowMultipleMasterLoads=1