Session replay is an add-on feature for paying customers only. You may try the feature for free for 14 days or 10,000 sessions (whichever comes first), after which you must contact Sales to continue use.
Heap will stop capturing new replays if your trial ends or if you hit your account limit. You can still watch replays that were captured, up to the retention limit.
Go to Account > Manage > Session Replay (under Plan) to check on your trial status or account plan limit.
If you are on a trial, there will be a banner that tells you if you have time left (or if it’s expired).
If you hit your session account limit, the Capturing status section will indicate that sessions are not being captured.