which gamecube mode do you want to use ?

DIOS MIOS, DIOS MIOS Lite, Devolution or Nintendont ?
you probably updated from an old version of USBLoaderGX, it used to have "MIOS" by default, OR you are still using an old version of USBLoaderGX released when ONLY SD with DIOS MIOS Lite was possible for gamecube games.

You are using "1268 v3.0c" ? that version DOES NOT exist.
it's either 1268, either 3.0c 1180.

I think you have 3.0c 1180, like I said, that version was released when there were no existing gamecube loader capable of loading from USB. Only SD was possible 3 years ago. since then, new gamecube loading method was released, and USBLoaderGX has been updated to support these new methods.
don't use old outdated version of the loader.
please, follow this to update your loader.

If you use this old version, I also recommend you completely delete your folder on SD:/apps/usbloader_gx/
do not keep your old settings or you'll have issues ! delete the full folder before following the guide.
then, you can look here, I explain how to setup USBLoaderGX with nintendont.
https://gbatemp.net/threads/trouble-connecting-nintendont-to-usbloadergx-on-vwii.515914/