This is the 14476th most frequent Korean word.
문제니까
"It’s because it’s a problem."
'문제니까' is used here to connect the reason (the problem is not difficult) with the advice (don't worry).
이 문제는 어렵지 않으니까 걱정하지 마세요.
Don't worry because this problem is not difficult.
'문제니까' indicates the causal relationship, explaining that due to the absence of problems, there's no need for concern.
그는 문제도 없으니까 걱정할 필요가 없어요.
He has no problems, so there's no need to worry.
'문제니까' is employed to indicate the reason for thinking about ways to resolve it (the fact that it's a problem).