Can an alias account approve a workflow request?

No. There must be a Google account associated with an email address to be able to sign into Form Workflow Plus application and approve a workflow request.

If you use other Google email accounts as aliases to send email from your main email address, please ensure that you are logged into the email address that is set up to approve a workflow request. If you cannot approve a workflow request please sign out of Form Workflow Plus app [https://app.formworkflowplus.com/#] and choose the correct account to sign in with.