This article explains what this error means: the account you are logging in with doesn't have access to the Provider Portal, most likely because the account isn't linked to a registration record.
Go to Security Manager, Users.
Search for the username of the account getting the error.
Check the Registrations tab.
To add a registration, click the Add Registration button and search for the registration name or ref number.
Tick the boxes for the areas of the portal that the account should be able to access.
Double-click the registration to open a pop-up window with the provider-level permissions.
Tick the area of the portal needed against the providers.
If you have checked and the permissions on the account look correct, this error can also occur if logging in with an email address, if the email address is also associated with a parent/citizen account. If the user has a parent/citizen account with their email address, they must always use their username and not their email address when logging in to the Provider Portal.