Conjugation of repartir for vous is repartez and not repartissez! Which is correct?

Are you sure it was «repartir» and not «répartir» ? As the lesson is on conjugation of -ir verbs that follow the 'regular' pattern, I suspect it was the latter, as it is mentioned specifically in the lesson. Check the quiz report, and if it is wrong, an issue report is the way to go.

2 different words and conjugations:

repartir - nous repartons/vous repartez (Follows the pattern of derivatives of partir)

répartir - nous répartissons/vous répartissez (Follows the 'regular' -ir verb pattern, shares derivation with répartition)

You can see  all the differences easily at Berscherelle conjugeur

https://bescherelle.com/conjugueur.php?term=repartir

It may help to know the difference:

Répartir (in this lesson) means to divide, distribute, or allot.

Repartir (the other) means to get started again or to leave again.