You can file an amended return for 2015 until April 15, 2019. I would expect that in most cases adding $10,000 of investment expenses would give you a large enough refund to make it worth the effort.
Investment expenses are an itemized deduction. If you did not claim itemized deductions on your original 2015 tax return (in other words, you used the standard deduction), you will have to enter any other itemized deductions that you are entitled to in order to get the full benefit of the additional investment expenses.
If you are subject to AMT (Alternative Minimum Tax) you will get no benefit from the additional investment expenses because the deduction for investment expenses is disallowed under AMT. If there is an amount on Form 1040 line 45 you are subject to AMT.
If your income is very high you might get little or no benefit from the additional deduction. (The same would be true if your income is very low, but that seems unlikely if you had over $10,000 of investment expenses.) You can prepare an amended return to see how much of a refund you would get, and don't file it if it turns out to not be worth it.
Go to the FAQ at the following link to access instructions for amending a 2015 tax return that you filed with TurboTax.
https://ttlc.intuit.com/questions/3410543-how-do-i-amend-change-or-correct-a-2015-return-in-turbotax
An amended return has to be filed on a special form, Form 1040X. It cannot be e-filed. You have to print it and file it by mail. It takes the IRS up to 16 weeks to process an amended return.
Investment expenses of more than $10,000 seems very high. Are you sure that what you omitted was investment expense, and not the purchase price of investments that you sold? Investment expense would typically be fees that you paid for investment management or investment advice. If you omitted the cost of investments that you sold, it's handled entirely differently, but it's even more likely to be worth filing an amended return.
I would also suggest that, if you are paying over $10,000 a year for investment management or advice, your income and assets are probably at a level where you should also be getting professional tax advice, and not preparing your tax returns yourself.
