Charles Ellicott Commentary 2 Kings 21:8

"neither will I cause the feet of Israel to wander any more out of the land which I gave their fathers, if only they will observe to do according to all that I have commanded them, and according to all the law that my servant Moses commanded them." — 2 Kings 21:8 (ASV)

Neither will I make the feet (foot) of Israel move (wander) ... — Compare the promise in 2 Samuel 7:10. The reference is to the migration to Egypt; and the thought is that the permanent possession of the Promised Land depends on the permanent adherence of the nation to Jehovah only.

Only if. — If only.

According to all. — Chronicles rightly has simply (to do) all; and so Septuagint, Syriac, Vulgate, Arabic here.

And according to all the law. — Omit and, with Chronicles and the Vatican Septuagint.