Users on a Descript Free plan are not able to purchase additional transcription hours beyond the 3 complimentary hours that are provided as a trial.
Once these hours have been exhausted you may upgrade to either a Creator plan for up to 10 hours/month per user of automatic transcription or a Pro plan which provides up to 30 hours/month per user.
You can upgrade your subscription by visiting https://web.descript.com/subscription and clicking the Edit button on your Drive Plan.