I received a Form 3921. I exercised, but I didn't sell any of them. When I inputted in TT, it raised my tax due by $18K. Do I have to input this form in my tax return?

My understanding was that if we didn't sell stocks, we didn't have to pay taxes on them.  So I'm not sure if I have to input this form on my tax return.  If I do, not sure why Turbotax calculated 18K in Taxes due.  Any help is much appreciated.