The source in where I studied told me that you mark the object of potential form with が particle instead of を particle because potential form is just a state, no actual action is taken. It specifically told me that を can be used but が is better. I know how to make sentences where I'm the subject. 山が登れない : I can't climb the mountain. 彼が言っていることが聞こえない : I can't hear what he is saying. But what if I want to say "He can't eat apples" or "Alice can't go to school" or even "Bob can't kill Tanaka". With が、を、は I could be mistaken in saying "Tanaka can't kill Bob" instead of "Bob can't kill Tanaka". Can が also denote the subject or only the object?
The particle you are looking for is に.
He can't eat apples.
Bob can't kill Tanaka.
He can't climb this mountain.
the reason why he can't climb this mountain
He can't hear what I am saying.
≒ 彼に私の言っていることは聞こえない。 (ga-no conversion)
Alice can't go to school.
(You cannot say アリスに学校に行けない. Probably because there is already に?)
(Different words are topicalized using は, but I don't know which is "better"...)
EDIT: changed the fifth example above for consistency.