Before you hit the web, type STYLESMANAGER into your AutoCAD command line. This will pop open the folder where AutoCAD looks for these files. If monochrome.ctb isn't there, you’ve found your problem.
✅ This CTB respects (LWT). If you need all lines to print with the same thickness, modify the CTB or use a monochrome STB. Autocad Monochrome Ctb File Download