These errors usually occur due to:
Once you have obtained the legitimate stdx603.ttf or stdx603.otf file, follow these instructions to ensure your CNC software recognizes it.
: Double-click the file and click "Install Font" in the Font Book window that appears. pairing this font with other typefaces for your specific project? Font Trouble "LT," "STD," vs. normal - Adobe Community
April 2018. STIX Fonts Version 2.0.0 Type 1 Postscript fonts released. STIX fonts ISO Fonts - PrintWiki