Are you not seeing the Facebook Page Role Invite in Notifications? Account Manager might probably have tried to cancel and resend the invitation. However, you are still not receiving a notification. In this case, follow our article further to find and accept the invitation.
PowerShell Commands to list, remove and add mailbox permissions of Microsoft 365
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To grant full access to an Microsoft 365 Mailbox you must first connect to the remote PowerShell, and provide MailboxPermission.