This usually happens when you have registered as an existing Basic-Fit member, while Basic-Fit does not recognise you as an active member. Are you indeed an existing member? Then please register using the transfer service or with the same email address as with Basic-Fit.
If you use a different email address than with Basic-Fit, you will not be recognised in their system, which is the most common cause.
Have you only used a trial month or day pass at Basic-Fit? Then you are not an existing member and you can simply register as a new member.
Has your application been cancelled for this reason? Then you can reapply, this time as a new member.