。
Effort is indispensable to studying.
Literal
Study [to-に] effort [topic-は] indispensable.
欠かせない is the negative potential of 欠かす ('to miss, to neglect to include'), meaning 'cannot be missed' = 'indispensable, essential.' It functions as an adjective in predicate or attributive position. The に marks the scope within which the effort is essential.