To those having the error when clicking on the tab in career mode, you need to open the series.ini file in the "series_formula3_V6_japanese_championship_2019" and fix the typo: REQUIRES=series_formula2_V6_japanese_championship_2019
it should read:
However if you want it to be unlocked for play immediately, just delete everything after REQUIRES=
and be sure to save the changes. This will fix it. Be sure to install the circuits used with this career set.