TurboTax did not have a way to enter box 1g (wash sale loss) from my 1099-B. This causes TurboTax to show a loss that I believe is not allowed. How do I fix this?

[baldietax]

at the bottom of the entry screen if entering "one-by-one" there is a checkbox "I have other boxes on my 1099‑B to enter" and you can enter 1g there

if entering in summary mode there is a check box "I need to adjust my total cost basis" and you can enter code W

Personally I always use one-by-one for these which will put these items on Form 8949.  if you enter adjustments at a summary level it will trigger the need to provide more details to the IRS via Form 8453.

if you have multiple sales (likely) but only some are wash sales you can split up the 1099B, enter the wash sales as one-by-one and the balance of your 1099B you can enter as summary level and it will be consolidated on Schedule D (check the totals add up the original totals for the 1099B).

[JamesG1]

If you are entering the information one a one-on-one basis, select the box I have other boxes on my 1099-B to enter.  Box 1g is available to report a disallowed loss due to the wash sales rules. 

If you are entering the information on a sales section totals basis, select the box I need to adjust my total cost basis.  W (nondeductible loss from a wash sale) is available to report a disallowed loss due to the wash sales rules.

The wash-sale rule prohibits selling an investment for a loss and replacing it with the same or a 'substantially identical' investment thirty days before or after the sale.

The loss not permitted is added to the cost of the newly purchased asset.  Your broker keeps track of the wash sales.

More information may be found here.

@rakvak