capital gains or loss tax

You would report the sale and any gain or loss in the year sold.  Thus if you sold it 2020, that would be the year you report it.

As to how much tax you would pay, that is all dependent on the gain (if any) and the amount of your other taxable income.

The important thing here is to establish your basis  If the land is inherited, (other than in 2010, special rules apply for that year), your basis should be the FMV on the date of death of the person you inherited it from.  Unless the estate used an alternative valuation date.  If you don't have the information, contact the executor of the estate to find out the value used in the decedent's estate. 

If you are selling the land for less than FMV, why?   If selling it to a related party, and there is a loss, the loss may not be deductible.

Basically you compute it this way:

Selling price of land                   $xx,xxx

Less basis                                   ($xx,xxx)

Less cost of sales

  e.g. RE commission, etc.       ($ x,xxx)

= gain/loss on sale                  $ xx,xxx

Here is a link to the IRS site and question about gifts & inheritances

https://www.irs.gov/faqs/interest-dividends-other-types-of-income/gifts-inheritances/gifts-inheritances

If you are selling the property to a related party for less than FMV, than there may be either a part gift involved and/or non-deductible loss.  That is something that needs to be considered when selling and reporting the sale.