Thanks for checking, KrisMc.

This might have to do with your user access for the QuickBooks Online account. If your access is a Standard or Regular user, that's the reason you don't have access to customising sales forms. Kindly check with the master/primary admin of the account to verify and have them make changes to your user access type.

You can refer to this link to learn more about the user access and roles in QBO: User roles and access rights in QuickBooks Online.

Message us back if you have further concerns.