Why was no one going in or out of Jericho before Israel's march around it?

Answer: It was shut up because of Israel

"Jericho was tightly shut up because of the Israelites. No one went out and no one came in" (Joshua 6:1).

“Now Jericho was tightly shut up because of the Israelites. No one went out and no one came in.”— Joshua 6:1 (BSB)