John Gill Commentary Deuteronomy 26:1

"And it shall be, when thou art come in unto the land which Jehovah thy God giveth thee for an inheritance, and possessest it, and dwellest therein," — Deuteronomy 26:1 (ASV)

And it shall be, when you are come in unto the land The land of Canaan, which they were now on the borders of, and just entering into.

which the Lord your God gives you [for] an inheritance which is often mentioned, to observe that it was not through their merits, but his gift, that they should enjoy the land; and the rather here to enforce the following law concerning the basket of firstfruits.

and possess it, and dwell therein not only had entered into it, but got the possession of it, and settled there. This shows as Jarchi observes, that they were not bound to the firstfruits till they had subdued the land and divided it; not as soon as they were in it.