“completely dispossess them.” The Hebrew text uses the figure of speech polyptoton (“many inflections”) for emphasis, using the word “dispossess” twice but in different forms. E. Fox (The Schocken Bible) translates the phrase “but dispossess, they could not dispossess them,” which is quite literal but awkward in English. Often the REV would put the word twice with “yes,” and read, “dispossess, yes, dispossess them” (see commentary on Gen. 2:16), but in this case, the negative “dis” followed by “yes” causes an incongruity and so “completely dispossess them” was the chosen translation.
The fact that the children of Israel did not completely conquer and dispossess the Canaanites shows a failure on their part to trust God and care enough about His agenda and what He wanted for them to carry out His commands completely. Typically human, the Israelites took enough land to comfortably (or mostly comfortably) settle in it and have flocks and farms, and think that was “good enough.” Many believers today act the same way, and ask God to help them until they get comfortable and then that is good enough for them. It does take a lot of effort to fully carry out God’s desires on earth, but for those who will go all the way with God and not stop when they are comfortable, there is great reward.