@nakov Do you have any idea how I could solve this?
If the teamID is not set in the request, an error occurs. (500 - "message": "Function name must be a string").
A required-rule should take effect during validation.
500 is a server error, might be because you are missing a value and the value after the last , is empty or I don't know what, you have storage/logs in each laravel project where you can find what the full error trace is. Maybe because you are using $request() which is not a function.. it should be without $ to use the helper function.
Now back to your validation, have you tried using the fluent API that I shared from the documentation above?