I discovered I was getting this error message when the IRS database didn't have ANY IP PIN on file at all.  If that's the case, the original 2020 probably didn't have an IP PIN# assigned yet.  Your client may be able to tell you what the first year was that they received a letter from the IRS about this.  OR,, you could try efiling without any and see if it gets rejected.  If it does, the irs.gov website should tell you about IP PINs for amended returns and which year to use..

I'm not a ProSeries expert....just another user, so if you still don't find the answer you need, try to get one of the experts in this community.