Put it on the board, talk to your team, and talk to your ETL. Not everyone checks the swap board and some ETLs will try to help your find coverage. Once someone does agree to take your shift, make sure they fill out their part on the swap board and make sure to tell your ETL so that it gets approved and updated in the system. Someone has to check for compliance before approving a shift swap so it's not always guranteed.
If you can't find coverage, you'll just need to call out and accept the consequences. ASANTS but some store hand out correctives if you call out on certain holidays. To call out you'll need to CALL the store at least 2 hours in advance (no you can't email or text someone) and ask to speak to the LOD. Let the know you won't be coming in. They'll ask you why and they'll pressure you to come in though. No matter what they say though, they won't be able to say you NCNS (which is much much worse than just calling out).