: It stores shape definitions for complex linestyles—such as lines with circles, squares, or custom dash patterns—so they render correctly in AutoCAD.

If the missing symbols aren't important to you and you just want the pop-up to stop:

Navigate to your AutoCAD installation's fonts folder, typically: C:\Program Files\Autodesk\AutoCAD [Version]\Fonts . the dgnlstyle.shx file into this folder. Restart AutoCAD and open your drawing.