The nearstars.stc branch uses the spectral type Y for several brown dwarfs, which hasn't been implemented in 1.6.2.
For that, either the code for class "Y" needs to be backported to 1.6.2, or CelestiaContent needs to have a new branch, called "1.6.2" where the class "Y" is replaced with "T9" as a workaround.
The nearstars.stc branch uses the spectral type Y for several brown dwarfs, which hasn't been implemented in 1.6.2.
For that, either the code for class "Y" needs to be backported to 1.6.2, or CelestiaContent needs to have a new branch, called "1.6.2" where the class "Y" is replaced with "T9" as a workaround.