@AymCTLI think it's bugged and Respawn is working on it. There is a report on Respawn's official "bug tracker". It says "fix in progress" so it should be fixed soon
https://trello.com/c/qHCchQcq/409-revenant-legend-not-unlocking-after-completing-challenges-until-game-is-rebooted
EDIT: didn't quite read your post before replying, so I don't know which challenges need to be completed to unlock Revenant, but It would not surprise me if the challenge goals keep increasing, so I guess that you have to just keep finishing them maybe. Otherwise I think he is unlockable with 12,000 apex tokens (the red currency). Hope it works out 😇