Combo products get their availability from the products they are based on. If one of your base products is sold out, your combo product will also appear as sold out.
You have one option to manage the availability for you combo product but that is by closing out days. Your base products will however still be bookable individually unless you close them out separately.
Select "Actions" > "Close outs" up in the right hand corner of your screen.
Select the date you want to close out from the calendar on the right hand side and toggle the switch next to the start time you want to close. The switch will as a result turn grey.
The date will become red once you've closed it out.
Note: If you have multiple start times, you can select the ones you want to close out individually or if you want to close them all, it's enough for you to toggle the top switch as it applies to all start dates.