My 1099 supplemental form shows a realized loss for the year. How do I capture that instead of the incorrect gains on 1099-b?

You are talking about stock transactions.

There could be Mutiple causes of a discrepancy, but the most common is the absence of cost basis.

1. Wages and Income
2. Scroll to Investments & Savings
3. Select Stocks, Cryptocurrency, Mutual Funds, Bonds, Other (1099-B) [Start or Revisit]
4. If you already have an entry, select the appropriate broker or add another account.
5. Select you broker or follow the interview.
6. As you enter the dates and proceeds, there is a check box for The cost basis is incorrect or missing on my 1099‑B
7. Use this option to correct any errors to cost basis.