。
She can't go to the party because she doesn't have a babysitter.
Literal
She [topic-は] babysitter [subject-が] not-being [because-ので] party [to-には] can't-go.
ので marks a softer, more circumstantial reason than から — 'given that.' The compound には combines the locative に (destination) with topical/contrastive は, narrowing the focus: 'as for the party (specifically), she can't go.' The negative potential 行けません ('cannot go') comes from the godan potential 行ける + the polite negative ません. ベビーシッター is a katakana loanword from English — culturally a relatively new concept in Japan, where babysitting professionalism is less established than in the US; grandparents and family networks more often fill the role.