Created a FGDB and Poly FC in Pro 2.1.3, created some poly's using clip of another feature layer. Output has 6 features, I want to merge the 6 down into 2. Select two of the features, Edit -> Modify -> Merge: The Destination layer type cannot be merged. Wut?!?! Further investigation reveals that "This feature layer is already locked by another running application" which is not true, as I just created it; it's on my local hard drive so no rogue user decided to start editing it; I only have one instance of Pro going, and one Catalog view going, and no running GP tasks. Closed Pro, finished the project in Arc, where merge from the edit toolbar worked flawlessly.