IRS claims additional tax owed on my 2018 return not showing a gain listed on Form 1099-B. Vanguard sent this to TTax but you omitted it from my prepared return. Why?

1099-B also requires Form 8949. This was also missing from my 2018 tax return. The IRS states that I had a gain of $14,670 from Vanguard's REIT Fund and that this omission results in an additional tax of $2,564. Can you explain why this (apparent) gain was not listed on my tax return you prepared or do you believe this data was not taxable?