Symptoms:

   Unable to change VM recovery VRA - getting an Exception: "This is not allowed when you change a datastore."

from the logs:
"
,E,248,ProtectionGroupController,ChangeRecoveryHostImpl,"Exception occurred during change recovery host implementation: Zerto.Zvm.Common.ProtectionGroupControllerException: VM [VM vm-12345, server 8e285c33-3572-4b0b-9b6d-fecc35d275b8] The recovery compute resource cannot be changed.
This is not allowed when you change a datastore.
"
     

Cause:

   it seems to be related to a previous data store change that was not updated.    

Solution:

Edit the VPG, but without making any changes.
Click 'Done' and wait until the VPG update has completed.
once done, reattempt the change of recovery host.