Did Abraham and Isaac possess the Promised Land when God made the covenant with them?
No. At the time of the covenant, Abraham and Isaac were still dwelling among the Canaanite tribes; the land was not yet theirs. God promised that the day was coming when He would give them the land, so possession was future, not present.
Les explains that in Genesis 26, when the covenant promise is repeated to Isaac, Abraham and Isaac are still dwelling among all the Canaanite tribes that were there when Abraham came in. They have not got the whole land, it is not theirs, and they are still dwelling in the midst of the Canaanites. But the promise is that the day is coming when they will not have to put up with the Canaanites, and God is going to give them the land. The covenant also repeats that God will give these countries to Isaac's seed and multiply his seed as the stars of heaven.