Shining Armor
Artifact — Equipment
Flash
When this Equipment enters, attach it to target Knight you control.
Equipped creature gets +0/+2 and has vigilance.
Equip 
(: Attach to target creature you control. Equip only as a sorcery.)
Eric Deschamps
The List (PLST)
• #ELD-29Rulings
Gaining vigilance any time after the moment you choose to attack with a creature won't cause it to become untapped.
(wotc • 2019-10-04)
If there are no Knights to attach Shining Armor to when it enters the battlefield, it simply remains unattached.
(wotc • 2019-10-04)
Rulings
Gaining vigilance any time after the moment you choose to attack with a creature won't cause it to become untapped.
(wotc • 2019-10-04)
If there are no Knights to attach Shining Armor to when it enters the battlefield, it simply remains unattached.
(wotc • 2019-10-04)