There could be a couple of reasons that can cause this, please try the below steps and see if they help:
Once you are provisioned for Target Standard and Classic both, you need to make sure whether you are added to the needed groups.
Kindly go to the Administration > Enterprise Dashboard > Groups from you Adobe Marketing Cloud account and see whether you are added to the Target Advanced and Standard Groups.
Once you are added to the groups, you need to link your Target Classic account to Target Standard via the client code.
If you are new please reach out to the other Admin for Classic and Standard both and he can add you as a user in Target Classic and give you the password to login.
If you do not know any one then upon following steps 1 to 3 please reach out to client care and they can help you with it.
