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@s-cain wrote:

The client did not receive a payment of principal or interest in 2021 but I can't enter a zero on line 21 of Form 6252.

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No payments equals no Form 6252.

It is technically not needed and is wrong, but if you are uncomfortable with the sale happening in 2021 and not mentioning it at all on the tax return, I suspect that your client would be okay if you reported that they received $1.

As a side note, I find it rather unusual that the sale happened and there were absolutely no payments at all (not even a down payment).  So you may want to double-confirm that with the client.