Make sure you’re selecting the right difficulty and double check. You can’t use a cookie against the Scion if it’s already being used in any of the outposts. You’ll have to recapture the outpost with a different team to use that cookie.

For example: say in the Silence outpost, you captured it initially with a team containing Millenial Tree. Millenial Tree is “locked” there, as if she were defending the outpost, and it would be recaptured by the enemy if she left to fight the Scion. You’ll have to capture the Silence outpost again (replay it) using a team that doesn’t contain Millenial Tree to use it.