The registration process requires two onchain transactions (Commit + Register) to be completed within 24 hours of another.

The "UnexpiredCommitmentExists" error means that the second Registration transaction is being attempted within 24 hours of the initial Commit transaction, however the browser may have lost track of it.

Solution:

Try disconnecting your wallet, close the page, then reopen Manager App (app.ens.domains), reconnect your wallet, and try again.