Sign-in doesn't finish. Make sure you're signed in on lichess.org in the same browser, then choose Sign in with Lichess again and allow EPC Chess to use your account. If you declined by mistake, just sign in again.
You were signed out. EPC Chess signs you out when its permission to use your Lichess account is about to run out. Sign in with Lichess again to renew it.
"No Lichess account linked" on a board. That player has never signed in to EPC Chess with Lichess (for example, they were added by hand with New player). The match can't start until they sign in with Lichess and join your school, or the coach takes them off the roster.
Lichess is down. Check lichess.org. EPC Chess can't create or read games while Lichess is unavailable; tell your coach.