Why did you receive this IRS letter? See this IRS website - https://www.irs.gov/newsroom/irs-has-begun-sending-letters-to-virtual-currency-owners-advising-them-to-pay-back-taxes-file-amended-returns-part-of-agencys-larger-efforts
See this website for further information - https://www.toptaxdefenders.com/blog/letter-6174a-and-cryptocurrency
If you did NOT report your gains and losses on the 2018 tax return then you Must amend the 2018 return to report those gains and losses.
See this TurboTax support FAQ for how to report - https://ttlc.intuit.com/community/cryptocurrency/help/how-do-i-report-bitcoin-or-other-cryptocurrency-as-a-capital-gain/00/27338
To amend a 2018 tax return using the online editions, see this TurboTax support FAQ - https://ttlc.intuit.com/community/amending/help/how-do-i-amend-my-turbotax-online-return/00/27577
To report capital gains or losses you will need to upgrade and pay for the Premier online edition or higher. Click on Upgrade on the left side of the online program screen when accessing the 2018 online tax return.