First of all double check if your most recent payment to Fitplan was successful. Check if you have received a receipt from Apple/iTunes, Google Play or Fitplan.

If your payment was successful, and your account still appear as inactive, then there's three possible ways you can solve this:

Option 3:

iOS Only-

Did you create your Fitplan account using "Sign In With Apple" -> Hide Email? This creates a brand new account with a proxy email that Apple gives you. Try logging in this way and see if you can access your plans.