What did Abraham not understand about the Abrahamic covenant concerning Ishmael?
Abraham did not understand the long-range scope of the Abrahamic covenant, specifically that the promised Messiah would come through his loins by the line of promise, not through Ishmael. He loved Ishmael and was grieved when Sarah demanded the bondwoman and her son be cast out, but he lacked the concept that the seed of promise was Isaac.
Les explains that Genesis 21 records the necessary separation of Isaac and Ishmael. Sarah saw the son of Hagar the Egyptian mocking and told Abraham to cast out the bondwoman and her son, because the son of the bondwoman would not be heir with Isaac, the son of promise. This was very grievous in Abraham's sight because of his son Ishmael. Abraham loved Ishmael, but Les points out that Abraham still did not have the long-range concept of the Abrahamic covenant—that a Messiah was coming through his loins, and that the promised seed would be counted through Isaac, not Ishmael. God then told Abraham to hearken to Sarah's voice concerning the lad and the bondwoman.
And the thing was very grievous in Abraham’s sight because of his son.