A permissions error occurs when someone who does not have full control of the Facebook business account attempts to sign up.
A common misconception is that you should be able to sign up if you are an admin of the Facebook page but, unless you have full control of the business account, you will not be able to complete the sign up process.
To resolve this issue, please forward the portal link to someone with full control of the business manager. Once they follow the steps to get signed up, any user with access to the Facebook page can use the portal.
A useful tip to find out if you have full control of the business account is to go to https://business.facebook.com/ and go to Users > People and check the Business portfolio access column on the table to determine if you have full control.