The problem is whether the plan will work out.

Literal

Problem [topic-は] plan [subj-が] well-go [whether-かどうか] [copula-だ].

うまくいく ('go well, work out') is an extremely common collocation — うまく (skillfully/well) + いく (go). かどうか ('whether or not') embeds the question as the predicate of 問題は.