If you were logged to the incorrect Artist account when signing up for a Sessions Event, you'll need to cancel your participation using the incorrect Artist account to free up the performance time. Then, after logging into the correct account, you should be able to see that performance time as available to pick. Please note that, in the meantime, somebody else might have already booked that exact performance time you've freed up, so we strongly advise you to double-check which account you're logged into before booking any performance times in Sessions Events.
If you need more information about canceling participation in events, see also: Help Article: "I want to cancel my participation at an event!".