合格 and 成功: passing and succeeding
Compare passing specified assessment criteria with an endeavor going well.
Passing a standard
合格 (ごうかく) means passing the criteria of an exam or assessment. 「試験に合格しました」 says that the speaker passed the exam. に marks the exam passed.
「合格を目指しています」 means aiming to pass. An aim is not an already achieved result.
An endeavor succeeding
成功 (せいこう) means that an intended endeavor or attempt succeeds. 「実験に成功しました」 says that the experiment succeeded.
「成功するとは限りません」 says that success is not guaranteed. It does not establish definite failure.
Choosing words for exams
For the official pass result of an exam, 合格 is usual. 成功 can give a broader evaluation of a study plan or undertaking, so the object and criteria differ.
「合格しましたが、希望した点数には届きませんでした」 means passing without reaching the desired score. Passing a standard and fully meeting a personal goal are not identical.
Does 「合格できるか分かりません」 mean the speaker has already failed?
Show the answer and why
No. It expresses uncertainty about being able to pass, not an established result.