Kotlin API - endpoint for users
I've two recommendations that probably have more to do with design rather than with code. The first one is to change the API path to players:

@RequestMapping("/players")


I personally like to pluralize resource path names even if it's widely discussed.

The other one is to modify your PUT endpoint in order to also accept resources to be created. That way you would have a single endpoint which accepts a list of players, where players might be new or already have an id. For the first case, the system must create them, for the second, update them. That's also a mather of disussion in the net.