Hello! Is it required to using marking particles?
What is the difference if I say:&nbsp; 어디 있어요? and&nbsp;어디에 있어요?

For me, these are same. I would use “어디있어요” in spoken, and "어디에 있어요” for formal, and written stuff. (I think “어디에 있나요” or "어디에 있습니까”(formalX2) are more formal way though")