Should it be ليست not ليس because شنطة is feminine?
Is that the best word for bag?
It's dialect (Egyptian). The MSA word is حقيبة (ḥaqiba), plural حقائب (ḥaqa'ib)
Would it not be more appropriate to stick to MSA?
They said on their website that they are doing a mix between MSA and dialect on duolingo Arabic