何の「[納期](https://eikaiwa.dmm.com/uknow/questions/17547/)」によりますが、こちらの文章が通じると思います。
「The due date has not yet been decided.」
「The deadline has not yet been decided.」
「The date of ~ is not yet fixed.」
「納期」= due date/deadline/date of ~
not yet = まだ~ない
decided/fixed = [決まっている](https://eikaiwa.dmm.com/uknow/questions/56296/)
「[納期](https://eikaiwa.dmm.com/uknow/questions/17547/)」= appointed day of delivery; delivery date ; date of delivery
「[定める](https://eikaiwa.dmm.com/uknow/questions/61438/)」=decided; determined
「未~」= un~; non~ QED undecided; undetermined.
「~は~です」= As for ~ it's ~