There seems to be some debate over whether or not Skeletons can be put to sleep by way of the Sleep spell in 5e.
Can they or not?
This limitation is defined in the spell description:
Undead and creatures immune to being charmed aren’t affected by this spell.
Skeletons are undead, therefore they cannot be put to sleep using the sleep spell.
They can't be put to sleep using the Sleep spell because of the inherent limitations of that spell. However, if you had another means of inflicting the unconscious condition upon them, for example a brass dragon's sleep breath, then they would be susceptible.