Operation must use an updateable query message

This is my query:

===============

UPDATE tblMortalityReview INNER JOIN qryInitialUnit

ON tblMortalityReview.EncounterNumber = qryInitialUnit.EncounterNumber

SET tblMortalityReview.InitialUnit = qryInitialUnit.Unit;

==============

Why it is not "updatable"?

Thanks

Operation must use an updateable query message