Charles Ellicott Commentary Psalms 139:6

"[Such] knowledge is too wonderful for me; It is high, I cannot attain unto it." — Psalms 139:6 (ASV)

Such ... —God’s omniscience is for man at once transcendent, unattainable, impossible. Possibly the article has dropped away, and we should read this knowledge. The Septuagint and Vulgate have “your knowledge.”

For the thought compare Psalms 139:17-18, and Romans 11:33.