Imagine that there is a mutant form of the protein securin that cannot be cut/degraded. How would that affect separase

It greatly affected the action of separase.

Explanation:

If there is a mutant form of the protein securin that cannot be cut or degraded, it greatly affected the action of separase because securin is responsible for the separation of two sister chromatids and this separation is only possible if the protein securin degraded. If the securin did not degraded, the separase is unable to separate the two sister chromatids which is necessary for the formation of daughter cells and then mitosis will not be completed.