Because you goofed. You sold the home in a previous year. You should have reported the sale in the year it happened, at which time you would either owe full repayment, partial repayment, or no repayment, and no further installments were owed.
To actually correct this you need to go back to the year you sold the home, and report the sale on form 5405 on an amended tax return.
Because this is the last year for repayments, it might just be easier to lie and say you still live in the home. You end up in the same place financially, unless you sold the home at a loss whenever you did sell it.