(1) DuoLiNxo, ليست literally means "(she) isn't". For example, سارة ليست موجودة means "Sarah is not here". You will see some usage of ليست in next skills.
(2) Christoph55835, we can say ليس or ليست. Both are acceptable -- والله أعلم.
To negate sentences with prepositions, you use ليس regardless of the noun. It's the equivalent of "there is not at me a skirt."
To negate "there is" sentences (with هناك ) you always use ليس ( ليس هناك ) so the same is done in the negative.