Business & farm

The TurboTax programs will not allow you to enter a negative cost basis for a 1099-B - it would appear there is a bad data field in one of your 1099-Bs. 

The only way to see where this error may be coming from is to print out the 8949, which requires you to pay for the program. You just want to pay for and print the form, not file. See the "Print Center option in the left menu of the online program (sample screenshot below)

A printout of the 8949 may indicate which 1099-B is causing the problem. If so, you should delete that 1099-B and re-enter it manually.

If the 8949 does not help identify the 1099-B  causing the problem, then your next option is to remove all the 1099-Bs and re-enter them manually.

See: How do I delete forms in TurboTax Online? 

After deleting the 1099-B(s), log out of the program. When you log back in, use the Delete a Form tool (List of Forms in My Return) to confirm it  (they) has been removed from your return.