The error message you are getting implies that you have at least 2 different Government Gateway Accounts, only one of which will have the enrolment for Self-Assessment online.
Any Government Gateway ID can be used to access your Personal Tax Account, but only one will give you access to your Self-Assessment.
If you are certain that the Government Gateway ID and password you are using are correct (ie the one you used previously to complete your Self-Assessment last year) then it is strange for you to be getting this error.
You mention filling in a CWF1 form - this would be to register as Self-Employed, which will not resolve this issue.
In order to solve your problem, you would be best contacting our Online Services Helpdesk:
Technical support with HMRC online services