Thanks for reaching out to us about this T2 submission concern in QuickBooks, @lsmpc.
You'll normally get the error 451 if the credentials entered in the options and settings do not correspond to the ones in the CRA records. For this, I want to get you to the right support to ensure you're able to recover your password to Efile the T2 form.
You'll need to reach out to the EFILE Helpdesk directly. You'll get their contact information from this link.
Please update us on how it goes. I want to make sure you're able to submit your taxes successfully to the tax agency. I'll be right here to share additional help also.