Impending Doom — ruling 1

If the enchanted creature can't attack for any reason (such as being tapped or having come under that player's control that turn), then it doesn't attack. If there's a cost associated with having it attack, the player isn't forced to pay that cost, so it doesn't have to attack in that case either.

Impending Doom : règles, fonctionnement et interactions (MTG)

Impending Doom 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

Enchant creature Enchanted creature gets +3/+3 and attacks each combat if able. When enchanted creature dies, this Aura deals 3 damage to that creature's controller.

How it works : Impending Doom

Original English text — Wizards of the Coast / Scryfall

#1January 24, 2020· Wizards of the Coast
If the enchanted creature can't attack for any reason (such as being tapped or having come under that player's control that turn), then it doesn't attack. If there's a cost associated with having it attack, the player isn't forced to pay that cost, so it doesn't have to attack in that case either.
No other card is named in this ruling.