You named the files wrong.
Print_GR_BL_Letters
Correction:
Print_Letters_GR_BL
Print_GR_Knife_TDM
I'm not sure what type of brick you want this, but if you want it for all print bricks:
Print_Letters_GR_Knife_TDM
If it's still not working, try this:
Print_Letters_GRBL
Print_Letters_GRKnifeTDM