You have several things wrong here. First of all when calling a method (no matter if it's a "normal" method or a generator method, an IEnumerator) you have to pass the parameters the method requires in it's declaration. Inside your Start method you don't pass a variable, it looks like you just copied the parameter declaration which makes no sense at all.
Your second problem is OnTriggerEnter is a callback that is called by Unity automatically. Usually it makes no sense to call it manually. Even if you want to call it manually you should make yourself clear what collider you want to pass to the function. Are you sure you know what you actually want to do? And if so could you please explain that in your question?
If you want to start the coroutine manually you have to call it like this:
StartCoroutine(OnTriggerEnter(someColliderReference));
where "someColliderReference" is the collider you want to pass.
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