It is the same as saying ‘我无法变成你’in Mandarin.

Sometimes people are very successful in some area. It is easy for them. They do not understand why you have so much trouble with that (what ever area it is.) You may get frustrated with their high expectations, so you remind the person that you are not as naturally gifted... "We can't be the same person" - or I can't be you - is one way you can do that.

If I say you "i can't be who you are " , it´s means that We are so different, and we can not be equals.

Sorry, I am not you, i can't be who you are.