Temur Charm
Instant
Choose one —
• Target creature you control gets +1/+1 until end of turn. It fights target creature you don't control.
• Counter target spell unless its controller pays 
.
• Creatures with power 3 or less can't block this turn.
Illustrated by
Commander 2020#230UncommonEnglishLegality
Rulings
You can’t cast Temur Charm choosing the first mode unless you target both a creature you control and a creature you don’t control.
If the first mode was chosen and either target is an illegal target when Temur Charm tries to resolve, neither creature will deal or be dealt damage.
If the first mode was chosen and just the creature you control is an illegal target when Temur Charm tries to resolve, it won’t get +1/+1 until end of turn. If the creature you control is a legal target but the creature you don’t control isn’t, the creature you control will get +1/+1 until end of turn.
If the third mode was chosen, creatures with power 3 or less that enter the battlefield after Temur Charm resolves also can’t block. A creature whose power was 4 or greater when Temur Charm resolved but is 3 or less as the declare blockers step begins also can’t block.