Hi,
the first instruction set was to leave the computer running whilst trying the process.
If it did not work I was suggesting:-
With Messages Open go to System Preferences > Notifications and turn them Off for Messages (None).
Quit Messages
Then Shut the computer down
Then:-
Restart Messages and restart Notifications
I do seem to have written rubbish in explaining that bit. Not sure why I wrote "or" instead of Restarting Messages (and Notifications).
The Link Resolve issues caused by changing the permissions of items in your home folder - Apple Support
will reset the Permissions of all the files in your Home Folder. (the one that has the Little House icon).
All the files in this Folder are supposed to be Owned by you as the account Holder and you should have Read and Write Permissions.
The Step 5 should also change the Permissions by the Apply to Enclosed bit bit this often does work for all the files.
The Terminal Command is more certain of doing so.
The Document is an Apple issued one.
Merging two Mac Users on a Computer can be done and should involve making the Owner of some files the same as the Account you are moving the Files into. The Read and Write Permission would also need to be done.
Whilst the process would have been similar it may not have involved all the files from one of the Mac User Accounts.
That said a small number will tend to lose Permissions in the normal course of things.
It is only when things don't work as they should that you might need to resort to this method in the linked Document.

8:48 pm Thursday; April 19, 2018
iMac 2.5Ghz i5 2011 (Sierra)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
iPhone 6 iOS 11.x and an iPad (2)