For me, this error results from the confusing TurboTax process of entering the 1099-K for ticket sales under "Other Common Income - 1099-MISC, 1099-K, 1099-G, tax refunds," then getting an error message to send you back to "Investment Income - 1099-B or broker statements" and then entering the cost basis information.
As a result of this confusing process, in the second step of entering my 1099-K, TT asked for "Enter the total proceeds for items sold at a loss or no gain." Since I had entered gross proceed without entering cost basis, I entered my gain here. Otherwise, there was no reflection of cost basis (at that point). That created my issue.
Even after I got the first error and went back to "Investment Income - 1099-B or broker statements" and entered the cost basis information, I still had had to fix this.
