When running the 1st sync for a Cloud only shared mailbox in a Hybrid Project the prepare fails even though the 1st prepare job which created the target ran successfully. You see the following error in the prepare logs
Failed to provision target MailUser for BTTestSharedMailbox@ukri.onmicrosoft.com, unable to license cloud based mail user.
This is because you are trying to create a cloud only object within a Hybrid project.
The 1st prepare created the target as an MEU and O365 cannot convert a MEU to a shared mailbox.
You need to reset the account in the P365 portal so its unmatched.
Then delete the target objects whioch have been created by P365
The target shared mailbox will need to be manually created in the target tenant.
Run a discovery so the source object can be matched to the new object in the target.