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-...
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-cryptocurrenc...
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