Hello! This seems to be more of an Exchange / AD infrastrucural question since the error message usually means that replication between AD and Exchange is not working properly. Or that the AD Schema does not have the correct information about your Exchange environment.
Please try and check these things to start with
1: That there are no replication errors from your domain controllers
2: Go to ADSI edit ->configuration partition-->services-->Microsoft exchange-->Under that you should be able to find out the "exchange organisation name". If this does not corespond to the domain of the user you are trying to create a remote mailbox for, or if this is empty, then you have trouble with the Exchange environment itself :)
3: Check that you are connected to a functional Exchange server in your Exchange Management shell
Feel free to DM or reply to me here if you need further assistance