Selling a house and inspector says the heat pump is on 30 amp breakers and needs to be changed to 20 amps. Why would this need to be changed?

Don't know. 12 AWG wire? Why not ask them. Their reports usually indicate why they made the recommendation

Ms. Wilbur...
I see a few options, one is to hire someone you trust (contractor) to determine if the inspector is correct,
Option two, is to do what he says as the issue now goes away.
A new breaker or two is likely much less than the contractor...

Brian is right.

There should be reason listed.

Check heat pump to see what ampereage is required.

Chuck