"상관없어" means "i don't mind"
if someone asks u to get your agree about behavior of someone, u can say "상관없어"
Eg.
someone:창문 닫아도 돼? (can i close the window?)
you:상관없어? (i don't mind)
and if u want to ask someone to get agree of someone,
u can say after your opinion or suggestion with question mark . "상관 없어?"
Eg.
you: 창문 닫아도 돼? 상관없어? or 창문 닫아도 상관없어? (do u mind closing the window?)
someone: 응 (no i don't)
but if someone asks u to do some action or choice, u can't use "상관없어"
because it is uncertain.
Eg.
뭐 마실래?(would u like something to drink?)
상관없어(wrong answer) 응,오렌지 주스(correct answer)