Ensure the texture path in the JCFG matches the actual file location. If your JCFG says "texture": "../ui/fonts/atlas.png" , but the file is in the same directory, the engine will throw a "failed to load jcfg font" error.
Use a tool like (Windows), TexturePacker , or Hiero (for libGDX). These tools take a standard TTF font and output: jcfg font