It is WAY too late to be filing amended returns.  Now you have to be careful not to miss the IRS notice allowing 30 days to request a CDP hearing.  Assuming the new owner is cooperating, get a letter from them (with complete name and EIN) explaining that they had bought the business by 2021 (had they?  You don't say when the transaction occurred) and that they reported the proceeds on their return.  

As @rbynaker describes, a lot of IRS mail would have gone to the old owner before this tax was assessed and then billed.  Maybe there was a change of address and the mail was never delivered.  At this point you can request an audit reconsideration (learn about those procedures) but the likely outcome will be to explain what happened in a CDP hearing.  If it's not too late for that.