On my last year return, I missed Schedule D for capital loss. Looking at my records, I have $6400 loss with $1400 deferred. Can I amend 2018 or adjust on 2019 return?

You will have to amend the 2018 tax return to report the investment sale.

See this TurboTax support FAQ for amending a 2018 tax return originally completed and filed using TurboTax - https://ttlc.intuit.com/community/amending/help/how-do-i-amend-a-return-in-turbotax/01/27579

How do you know how much of the loss is deferred if you didn't report the loss on your tax return? When you say "deferred" I assume you mean that part of the loss is carried over to the following year. But I don't see how you would know the amount of the capital loss carryover if you didn't enter the investment sale in your 2018 tax return, since it is affected by other things in the tax return. Or did you mean something different by "deferred"? If you are calculating the amount of the capital loss carryover without actually entering the sale in your tax return, you might be making some assumptions that are not correct.