The data is in the website (link is in the question), there's no information missing.

Find the average annual number of fatalities per million passenger enplanements. (An enplanement is defined as a "a revenue passenger boarding an aircraft.") This can be found in Table 3, "Passenger Injuries and Injury Rates, 1999 through 2018, for US Air Carriers Operating Under 14 CFR 121" in the NTSB spreadsheet "1999-2018 Preliminary Aviation Statistics."

This can be downloaded here:

https://www.ntsb.gov/investigations/data/Pages/aviation_stats.aspx.

(Note, this data only relates to passengers, not including the crew, and is only for large aircraft operated by airlines and cargo carriers. It is limited, but the most appropriate for this analysis.) Report the average annual number of fatalities and the average annual number of enplanements (in millions), averaging over all of the data available (1999-2018). Calculate the average annual number of fatalities per million enplanements, averaging over all of the data available (1999-2018), and report that estimate.