。
You shouldn't leave the problem unresolved.
Literal
Problem [obj-を] unresolved state [with-のまま] keep-[ておく]-[nom-のは] no-good.
未解決 ('unresolved,' with 未- 'not yet' prefix) is a stock term. ~のまま ('in the state of X, as X is') expresses maintaining a condition unchanged. Combined with ~ておく, this gives 'leave (it) in the state of X.' Multiple compound concepts in one sentence.