John Gill Commentary Deuteronomy 31:7

"And Moses called unto Joshua, and said unto him in the sight of all Israel, Be strong and of good courage: for thou shalt go with this people into the land which Jehovah hath sworn unto their fathers to give them; and thou shalt cause them to inherit it." — Deuteronomy 31:7 (ASV)

And Moses called unto Joshua
Who might be at some distance from him, with the tribe to which he belonged. The Targum of Jonathan adds, "out of the midst of the people:" and said unto him, in the sight of all Israel ;
now assembled together, and what follows was said in their hearing, to make him the more respectable to them:

be strong and of a good courage ;
the same that is said to the people in (Deuteronomy 31:6) , and which was still more necessary in him, who was to be their general, and to go at the head of them, and lead them on to battle; and though Joshua was a man of courage and valour, as well as had military skill, as appears by his fight with Amalek, (Exodus 17:9–13) ; yet such an exhortation was not needless, seeing he had so much work to do, and so many enemies to contend with:

for you must go with this people unto the land which the Lord has
sworn unto their fathers to give them, and you shall cause them to
inherit it ;
the Targum of Jonathan is, "which the Word of the Lord has sworn to give;" the land of Canaan, thither he must go with them; this was the will and determination of God, and he must go alone without him, Moses, which would be a trial of his courage.