RE: MPOS Activation Error: The configuration key is not valid
I do not expect you to change anything else other that what I outlined in the above message, so, you should not change Transaction Service Azure Resource.
Assuming:
a) MPOS and Store Commerce are pointing to the same Retail Server
b) You replaced Retail Server's configuration file attribute tenant-Id from ADFS to Empty Guid
c) MPOS device activation successfully completed
It is not possible you are getting the same error as before with Store Commerce. Please provide the details, especially correlation ID if there are any, such as AppSessionId or Activity Id for instance.
Also, given you have direct access to your Retail Server, have a look into its logs in Event Viewer to see if there are warnings/errors which can help you narrowing the error down.
