If you’re on Windows it must be “Courier New” and if the font you set is not available on the system it will revert to System.
This is my “base installation safe” monospaced constant:
#tag Constant, Name = kMonospacedFont, Type = String, Dynamic = False, Default = \"", Scope = Public
#Tag Instance, Platform = Mac OS, Language = Default, Definition = \"Menlo"
#Tag Instance, Platform = Windows, Language = Default, Definition = \"Courier New"
#tag EndConstant
If you have recommendations for Linux, I’m all ears!