If my answer does not answer your question, please give me more details so I can help you. Thank you.

What investment expense?  Box 2a is income you received even if you reinvested it.  

All Miscellaneous deductions subject to the 2% limit, including unreimbursed job expenses (reported on Form 2106) have been repealed for tax years 2018 – 2025.

Deductions that have been suspended for 2018
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FAQ on 2018 changes <a rel="nofollow" target="_blank" href="https://ttlc.intuit.com/questions/4482394-how-will-tax-reform-affect-my-2018-federal-tax-return">htt...>

If you mean, where do you enter investment expenses related to generating this capital gains income? 

Deducting investment fees and expenses has been suspended for tax years 2018-2025.

• Under the Tax Cuts and Jobs Act (TCJA), Miscellaneous deductions subject to the 2% limit, including unreimbursed job expenses (reported on Form 2106) have been repealed for tax years 2018 – 2025.

For more information, see Which federal tax deductions have been suspended by tax reform?

The box 2a amount is fully taxable (as a long term capital gain). You do not get to deduct your cost basis (investment expense) because you have none. You cost basis is reflected in the reduce price (NAV) of the shares in the mutual fund.

Just enter the 1099-DIV, exactly as received. TurboTax will handle it properly, entering the capital gain distribution on line 13 of Schedule D (or, less frequently, directly on line 13 of form 1040/schedule 1, with the Schedule D  not required box checked)