I sold it in 2019 and don't know what to report for the cost of the land?
Assuming you sold at a gain, any amount over the cost basis of the land (which I assume is zero) will be fine. Since you did not properly allocate 17 years ago, it's to late now. You simply have to recapture all prior depreciation and pay taxes on it in the tax year of the sale. You will be taxed on that recaptured depreciation up to a maximum of 25%.
I don't know what kind of issues this will cause for you with the IRS down the road 24-36 months after you file reporting the sale, if your overall AGI puts you in any tax bracket above the 24% bracket. So professional help may be something you want to seriously consider; especially if your state taxes personal income.
