Your marked as "busy" on your Calendar
Where the event is supposed to be you may have an event to "hold the spot" or you may have an all-day event setup that is not allowing for Calendly to provide an open date.
Solid left lines beside the event are indicators your calendar event is marked busy.
Date Range is Set Wrong
Depending on how your event was setup, you may not have the correct date range or future access.
It's not listed in the times available calendar