Ad Nasuem

You attempt to copy a creature in the process of casting a spell. If the creature is casting a spell of 4th level or lower, If it is casting a spell of 5th level or higher, make an ability check using your spellcasting ability. The DC equals 10 + the spell's level. On a success, you copy the spell

At Higher Levels: When you cast this spell using a spell slot of 4th level or higher, the copy spell works if its level is one more spell slot higher to the level of the spell slot you used.