The correct place to enter a cost basis adjustment is on a different screen than the one you are using.
In the "Tell us about your Box __ sales from your 1099-B" screen, you need to check "This sale involves an employee stock plan (including ESPP) or another uncommon situation."
The next two screens will allow you to enter the correct adjusted cost basis (not an adjustment to the reported basis).
Use the attached screen shots as a guide.
I am trying to do this but I can't get to the part of the interview for the Tell us about your sales from 1099-B". It came up once before I found this thread and I clicked no and now I can't get it back even if I cancel the amendment and try again. I am having to do an amendment because I already filed and I got this K-1 subsequently.
@jackkan At your original 1099-B entry in Stocks, Bond, etc. choose Edit.
As you go through the screens you will come to a screen where you can indicate you need to Adjust the Cost Basis (screenshots for TTO and Desktop shown) and enter the correct Cost Basis (not the adjustment amount).
If you can't find these screens, you may want to delete your 1099-B entry and start over.
Click this link for more info on Entering K-1 Cost Basis Adjustment.