I am not an attorney, CPA or a REALTOR. However, I assume you have a deed restriction on a property that says you can’t sell it. That does not make the value 0 however. Without knowing more about how you obtained the property I would question that a perpetual “Never be sold” is a complete gift and sounds like control from beyond the grave. There are people who can value a controlled property but I would be asking for a legal opinion as well about the restriction.

On this thought, I am not sure I would see the downside of listing full market value of the property on your 990.