An Identity Protection PIN (IP PIN) is a six-digit number that prevents someone else from filing a tax return using your Social Security number. The IP PIN is known only to you and the IRS and helps to verify your identity when you file your electronic or paper tax return. If you received the IPPIN because you were a victim of ID theft, the IRS sends the new PIN to you each year via the US Postal Service.
Starting in 2021, you may voluntarily opt into the IPPIN program as a proactive way to protect yourself from identity theft. See this link for more details: https://www.irs.gov/identity-theft-fraud-scams/get-an-identity-protection-pin
If you are getting a rejection for IPPIN and you don't have yours, you can attempt to recover at this site. https://www.irs.gov/identity-theft-fraud-scams/retrieve-your-ip-pin
Once you have your IPPIN, you will need to go into your appropriate year tax return, then click on My Info, then Taxpayer Info, you will see the space to enter it just under where you have entered your occupation. Once you have this updated, then you can resubmit the return.