Upvote:1
If you're flying with TUI, online check-in opens 14 days before your flight takes off. You'll need to check in for your outbound and return flights separately. Online check-in for your outbound flight is available right up until three hours before take-off and, for your return flight, closes six hours before take-off.
So if your return flight is more than 14 days away, online check in will not yet be available. You will be able to check in when your flight is less than 14 days away. This may be once you are already in Spain.
Note that if you are referring to the problems at UK airports, these largley relate to shortages of security staff and while obviously checking in online gives one less thing to worry about, it won't help you with queues for airport security. If the airport you are travelling from is known to be having difficulties, all you can do is arrive earlier than you might normally do.