What method are you using to sign in? If you are using the same credentials as Analytics, you should be using the Standard login. For marketing Cloud, use the Marketing Cloud method.
Also, it could be very well a case where you do not have the permission to access Report Builder. In this case, you will need to reach out to an Administrator to grant you access. If you are an admin, you can grant yourself the rights.
Ref: User and Group Management
