Charles Ellicott Commentary 1 Chronicles 21:29-30

"For the tabernacle of Jehovah, which Moses made in the wilderness, and the altar of burnt-offering, were at that time in the high place at Gibeon. But David could not go before it to inquire of God; for he was afraid because of the sword of the angel of Jehovah." — 1 Chronicles 21:29-30 (ASV)

A parenthesis, relating why David did not instead go to the ancient Tabernacle, which then stood at Gibeon. (Compare to 1 Chronicles 16:39–40.)