I want to discuss the importance of keeping the appointment once it has been set. Once you have both committed to this valuable event, cancelling or postponing it can give the impression to your team member that their work and effort is not important and therefore doesn’t warrant evaluation. Which in turn can lead to them becoming demoralised and a lack of engagement in the tasks assigned to them.
Additionally it can give the signal to the wider organisation that you don’t have the capacity to prioritise your teams’ development. Two situations I am sure you would rather avoid.
There are always exceptions and I would say in this instance the only exception would be if your team member didn’t complete their pre-appraisal preparation. I have postponed appraisals before because my team member had failed to do the required preparation. If they don’t commit to this work it can make the appraisal less effective. However a small caveat, if the person in question uses this as a delaying tactic because they do not want to have an appraisal, don’t postpone the meeting.