Answer

The portion of The Assumption of Moses cited by Jude has been lost. The Catholic Encyclopedia article on "Apocrypha" explains,

Aside from a few . . . brief references in patristic literature, nothing more was known of this apocryphon until the Latin manuscript containing a long portion of it was discovered by Ceriani in the Ambrosian Library, at Milan, and published by him in 1861 . . . The manuscript breaks off abruptly at chapter xii, and the portion cited by Jude must have belonged to the lost conclusion.