On the sessions list, you can view session replays by clicking the right-arrow button under Actions for each session. You will occasionally see a grey play button instead for some sessions.

These are session replays that you are unable to play, either because the journey was not able to be recorded, or because the browser on which the journey occurred is not supported for session replay recording, or due to the fact that the replay has been deleted as per your account's retention settings.

Hover over the play button to see the relevant message as to why you cannot play the session replay.

You can also use the session filters to only show playable sessions by ticking Show replay only.