You decide whether to have the targeted creature block Giant Ambush Beetle this turn if able at the time the "enters" ability resolves, not at the time Giant Ambush Beetle attacks.

Deciding to have the targeted creature block doesn't force you to attack with Giant Ambush Beetle that turn. If Giant Ambush Beetle doesn't attack, the targeted creature is free to block whichever creature its controller chooses, or block no creatures at all.

Tapped creatures, creatures that can't block as the result of an effect, creatures with unpaid costs to block (such as those from War Cadence), and creatures that aren't controlled by the defending player are exempt from effects that would require them to block. Such creatures can be targeted by Giant Ambush Beetle's ability, but the requirement to block does nothing.

If you choose to have the targeted creature block but that creature isn't able to block Giant Ambush Beetle (for example, because the targeted creature can block only creatures with flying), the requirement to block does nothing. The targeted creature is free to block whichever creature its controller chooses, or block no creatures at all.