Hey @Tyrion1803 yes this can be annoying..!
The issue is that in the "GSN-HydroCel-129" montage, the reference channel is named "Cz" (not "VREF"). If you do print(montage.ch_names) you can see what I mean.
MNE-Python can handle that specific mismatch, if you do:
raw_data_t1.set_montage(montage, match_alias=True)
and that should fix your problem!
Scott