Usually, you will meet below two errors when try to log in Couch to 5k using Android system. It usually happens if there are some changes on server. 

Error Message: z

If you met the error message z, please try the following two ways to login with Facebook account.
  1. Log out Facebook and try again:
    1. Log out Facebook native app
    2. Go to phone Settings > Apps & notifications > Couch to 5k
    3. Tap FORCE STOP
    4. Launch Couch to 5k
    5. Sign in with Facebook account
  2. Uninstall Facebook and try again:
    1. Uninstall Facebook
    2. Launch Couch to 5k
    3. Login with Facebook account
Error Message: -24

This is an error with Google Play. Please go to http://www.digitalreborn.com/unknown-error-code-app-install/ for further help. If it doesn’t work, go to the Google Play support page: https://support.google.com/googleplay/troubleshooter/4592924?hl=en&ref_topic=3365284