The teambuilder is likely saying it's illegal because Chimechite wasn't introduced until Legends: Z-A, a game which doesn't contain Toxapex, so it's a combination that can't actually exist in the real game. This is just a feature that's helpful for other formats but isn't very relevant to the one you're trying to play.
If you set the team's format to Mix and Mega, the team will still successfully validate and you'll be able to use Toxapex with Chimechite in a M&M battle without any issues, so just ignore that warning.
