Dragging emails between folders gives me Cannot move the items. The item could not be moved. It was either already moved or deleted, or access was denied.
Happens on random messages in an IMAP account on Outlook 2016. The same messages move fine from webmail.
On IMAP this error means the local cache and the server disagree about the message. Outlook thinks it holds an item the server has already changed or removed, so the move is rejected.
Two quick fixes cover most cases. First force a full sync of the folder by clicking into it and pressing Shift+F9. Second, if a specific folder stays broken, clear its offline copy: right click the folder, choose Clear Offline Items, then click Update Folder on the Send / Receive ribbon.
If the error hits every folder, the OST cache itself is unhealthy. Close Outlook, rename the .ost in %localappdata%\Microsoft\Outlook and let Outlook rebuild it from the server.
One more cause worth checking on POP or local PSTs: the destination data file is at its size cap. A full PST rejects incoming moves with this same message. Check the file size and archive or split it if it is near 50GB.
Clear Offline Items on the two problem folders fixed both. Everything moves normally now.
Adding for future readers: I had this on a PST that had hit 50GB. Splitting the file with the Archive feature stopped the error instantly.