Error Code:

Error Code	16003
Message	The user account does not exist in the directory or the user hasn't been explicitly added to the tenant. To sign into this application, the account must be added to the directory.
Remediation	Invite the user to the tenant, or ignore this error if the user isn't supposed to be a member of the tenant. This can be the case when someone is sent to a login URL for your tenant without being a member, or picks the wrong user account.