when you sold the house in 2014 the unrepaid balance was due in that year. thereafter there would be no repayment and no form 5405
When You Must Repay in Full
The credit must be repaid in full, in one lump sum equal to the balance, if you sell a home that was purchased in 2008 at any time within the 15-year repayment period. This involves preparing and filing Form 5405 which will calculate how much you owe.
Have you been making the $500 payments every year since you sold the property ?
If so that was soooo wrong and you need to correct this error now. The easiest way is to just say you sold in 2020 so you pay back the balance due now on the 2020 return. The correct thing would be to amend the 2014 return and take credit for all the other payments however this is a pain at best and will not accomplish anything different in the end.