Guys somebody asked me if Jinx(manhwa) is a rape? Explain why you said Yes or No.

Yes.
Anything but enthusiastic consent is sexual assault, meaning consent under manipulation, coercion or force is assault
Dan didn't walk into that contract on equal ground, JK took advantage of his situation and cornered him into accepting. Therefore their agreement on itself is coerced
Now, even if anyone were to argue otherwise, it would still be assault bc if you go read it, EVERYTIME he was said "no" "stop" "wait". ""Oh but all bottoms do that in BL"" yea and just bc it's normal doesn't mean it's not assault
Even that one time when Dan full on walked into JK's room to be fucked, he was drugged and incapacitated

Imo, it's rape.
some people might argue cause they did agree to do it for money and dan's grandma hospital bill. But you can't ignore those time where dr dan clearly doesn't want to do it but jaekyung still insist, therefore, rape. I read that one a long time ago but there's one in the gym's SHARED bathroom, while OTHERS were there, and dr dan CLEARLY don't want to do it there.. it's rape,no? And another on the locker room(?) I don't remember well, but dr dan also don't want to do it there.

No, because it really depends on one's definition of rape. In the fictional smut BL world, it's the standard push-and-pull plot—where the seme is a dumb brute, but in a sea of red flags, Jaekyung is still a saint ╮(￣▽￣)╭. There are also slap/kiss fictional dramas.
In the real world (US-centric view), yes, this is considered rape because "no" means "no," and we don't romanticize rape in our movies. In real world (Europe), specifically Italy, a judge once ruled that a woman wearing tight jeans couldn't be raped—essentially blaming the survivor. Every culture has its own perspective, some people like aggressive male lead, while some people like gentle green flag lead. Some people like a cute Cinderella waiting for a glass slipper, some people like a bossy Katniss standing her ground against the Capitol. For your friend, it could be rape or not, depending on how culturally familiar he or she is with the BL world dynamics.