>>>>>this issue occurred because when starting Autocad I had selected the "acadISO - Named Plot Styles.dwt" option.

Yes, in AutoCAD you can use two different types of Plot Style tables: color-dependent and named.
Most users (and me too) prefer to use Color-dependent plot style tables (CTB).
You can read more about Plot Style tables in AutoCAD here:
http://help.autodesk.com/view/ACDMAC/2021/ENU/?guid=GUID-2FD01085-DDD8-49D2-A910-E63EA45A7FED