If divorce is a sin, then why does the catholic tribunal require it done in order to get an annulment?
It is important to remember that divorce is not always sinful.
2383 The separation of spouses while maintaining the marriage bond can be legitimate in certain cases provided for by canon law.
If civil divorce remains the only possible way of ensuring certain legal rights, the care of the children, or the protection of inheritance, it can be tolerated and does not constitute a moral offense.
As to why divorce is required, there’s a relatively simple answer. Canon Law (canon 1676) requires tribunal judges to be sure that reconciliation is not possible between the spouses before proceeding with an annulment case. Obtaining a civil divorce is a sign that reconciliation is not possible.