The passcodes are built into the 'Join' links from the Canvas page. You should not have to enter a passcode.
Return to your Zoom class link and try entering the class again.
Only @my.acm.ac.uk email addresses will be able to sign in, and you should receive a warning if you try to use a non-ACM account.
You may need to register for classes. This is just like taking attendance. Use your ACM email address.