The basis on the 1099 won't be correct. You'll want to calculate it following the instructions on the Sales Schedule, and then enter the calculated value on the 1099. Here's a link to a post that walks through how to make this work correctly in TT: https://ttlc.intuit.com/questions/3760966-how-i-report-the-sale-of-mlp-shares-in-turbo-tax-i-sold-al...
Basic question on sales schedule. Purchase Amt: $20000 Adjustment to Basis -($32000) Is my basis -$12000
Never could figure out subtraction a negative number
Unfortunately, you have an additional problem.
First the math: your basis would be -12000.
However -- the problem -- your basis is not allowed to go below 0. The fact that it is now means that, during some year in the past, it hit 0. In that year, you were supposed to start filing differently. My suggestion is to get someone qualified to look at your past K-1s to figure out if any past years need to be amended and how it affects your sale, because its not something that can be worked through over the forum. But in a very complicated nutshell:
Your basis is the sum of your capital account (Section L) and your nonrecourse loans (Section K). When this is greater than 0, and your capital account is greater than 0, you file as normal. This is how PTPs start for everyone.
Over time, your capital account falls. In the year your capital account falls below 0, but you still have a positive basis (because of those nonrecourse loans), you no longer have capital at risk. That's a different checkbox in TT, brings in different forms, and brings in different complications upon sale.
Then, in the year your basis actually goes below 0, you have to start paying taxes on the differential to keep it at 0. Different filing issues. Different issues at sale.
Sorry I can't be more help on this.