"法律上的效力" can be translated into English as "legal validity" or "legal effect".
"Legal validity" emphasizes the binding and enforceable nature of a law, regulation, or legal document within the legal system.
"Legal effect" focuses more on the consequences or outcomes that a legal provision, act, or decision produces within the legal framework.
Both expressions are commonly used in legal contexts, and the choice between them may depend on the specific nuance you wish to convey. In most cases, "legal validity" is a more straightforward and commonly used translation for "法律上的效力".