I happened to run into this error message and it totally failed Google search so I figured I'd post the issue and solution here for the next person.

Attempting to run the DAOS compact on a database after enabling DAOS for the server generated the error: "Unable to compact attachments into DAOS, database must be upgraded to R85 or later database version" even though the database was already in R85 (ODS 51) format.

Turns out that in an earlier server rebuild the CREATE_R85_DATABASES parameter had not been set in notes.ini.  So even though the ODS was already 51, the compact was actually trying to set the database ODS back to 43 (R6), and then was unable to enable DAOS for it.  Once we added the parameter back in we were fine.