No, the online check in is optional. You can show up without even checking in at least 90 minutes before departure. There might be a port which enforces check in times (don't think Barcelona is one of them), in which case it would be good to pick a time.
Check in online should open 45 days before sail date around midnight Barcelona time. Check in at the pier must be done no later than about 90 minutes before departure.
In general, it's better to use the browser based online planner for pre-cruise purchases. Usually that's done on a bigger screen where you can see a bit more of what you doing/buying and usually works better than the app.
The 24 hour package is NOT available in the planner - only on board. It is typically about $3-4 more than the on-board full sailing per day rate. Voom package on board pricing is usually fairly close across the fleet (within $1-2).