Answer:
The anaphase of the Mitotic phase
Step-by-step explanation:
During the anaphase, the proteins (cohesins) that bind sister chromatids together are cleaved to produce two identical daughter chromosomes (meaning each chromosome contains the exact copy of the same genetic material). These identical chromosomes are then separated by a structure known as mitotic spindle. Individual mitotic spindle attaches itself to each chromosome which are then pulled to opposite poles/"sides" of the cell.
Thus, if an enzyme is going to prevent separation of chromatids during mitosis, the enzyme would do so at the beginning of this process, preventing the cleavage of cohesins which would prevent the separation of the chromatids and subsequently prevent this anaphase from occurring.