Giant Ambush Beetle — ruling 1

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.

Giant Ambush Beetle — ruling 2

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.

Giant Ambush Beetle — ruling 3

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.

Giant Ambush Beetle — ruling 4

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.

Giant Ambush Beetle : règles, fonctionnement et interactions (MTG)

Giant Ambush Beetle is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Haste When this creature enters, you may have target creature block it this turn if able.

How it works : Giant Ambush Beetle

Original English text — Wizards of the Coast / Scryfall

#1May 1, 2009· Wizards of the Coast
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.
No other card is named in this ruling.
#2May 1, 2009· Wizards of the Coast
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.
No other card is named in this ruling.
#3May 1, 2009· Wizards of the Coast
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.
No other card is named in this ruling.
#4May 1, 2009· Wizards of the Coast
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.
No other card is named in this ruling.