Viscous Mockery is a cantrip, so it doesn't require a spell slot, regardless.

Enthrall, on the other hand is a second level spell. This would *normally* require a spell slot to cast. But, as it is stated in the trait description:

> [...] When you reach 5th level, you can cast the enthrall spell once with this trait. You must finish a long rest to cast these spells once again with this trait.

There is no mention of consuming spell slots, which means that you can do it regardless of spell slots - you do not need to even be a caster to use this ability (and the Bard class, nor the Sorcerer class have access to this spell, regardless).