Where can I record a net capital loss (arising from stock market investments)?

Do you have a 1099B for sales?  You have to enter each sale and Turbo Tax calculates the net gain or loss.  You need to use the Premier version to enter investments.  You don't enter gains or losses in IRA, 401K of retirement accounts.

You can enter a 1099B manually or try to import it.  If you import it be sure to check it over close and make sure the cost basis got imported and is right.  How to import your 1099

https://ttlc.intuit.com/community/entering-importing/help/how-do-i-import-my-1099/00/26254

Enter a 1099B  under

Federal Taxes Tab or Personal (Home & Business)

Wages & Income

Then scroll down to Investment Income,

Then Stocks, Mutual Funds, Bonds, Other - click Start or Update