Where do I enter a 1099 B capital loss?

In the Online version you need Premier or higher.

Enter a 1099 B under
Federal Taxes on left or Personal (Home & Business)
Wages & Income
Then scroll down to Investments and Savings
Then Stocks, Cryptocurrency, Mutual Funds, Bonds, Other (1099-B) - click Start
Then click Add Investments at the bottom
Next screen hit Continue

Then don't enter or pick a bank, at bottom click on Enter a Different Way
Then pick box Stocks, Bonds, Mutual Funds - then Continue
Enter the bank or broker
Answer some general questions and Continue
Then Continue a couple more screens and you finally get to the 1099B entry screen.