How To Remove The Contest Tab From Your Facebook Page July 07, 2021
Facebook no longer supports removing third-party Page tabs. Facebook engineers are aware of the problem as mentioned in this bug report:
Unfortunately we don't know any workarounds at the moment. Sorry for this inconvenience. I will raise this with the engineering team again as it appears to be affecting a significant number of developers.
Facebook has announced a new Pages experience a few months ago and they are gradually rolling it out. All pages should be transitioned to this new design by the end of the year. The redesign does not support third party page tabs and so the contest tab will automatically be removed when your page is migrated.
We know how frustrating this is and we are sorry we cannot help. The ball is in Facebook court and it seems it is going to take a bit of patience.