question, why is it such a big deal to call someone ‘hyung’ in a game?

I don’t think it’s bad I think it just means you know them on a personal level

Whats the problem w that,they make it sound like a big deal in this manwha

If I remember they said it was against the guilds rules

I think it's because in most countries in Asia, people from different age groups call each other with honorific because we're big in seniority. Most of the time, if they're only someone you know from random places/work/school, we would use terms that means senior like sunbae/senpai or in Korea (from my limited knowledge) they uses -ssi for like "Mr." or "Ms." like how Japanese uses -san at the end of someone's name. Calling someone with hyung/unnie means you're closer to them past the "professional" level or in a more intimate way.

At least that's just from what I know (￣∇￣") I'm Asian but not from Korea so I might be wrong

it’s against the guild’s rules

Yeah it's against their guild rules. Like being too close personally with the guild members. They mentioned it when Jistar is pestering Neutaa to call him hyung.