: error: undefined reference to `usb_open'

This error message typically occurs when you are trying to compile a program that uses the libusb library, but the linker cannot find the definition for the `usb_open` function.

To resolve this error, you need to link your program with the libusb library. You can do this by adding the `-lusb` flag to your compiler command:

gcc -o myprogram myprogram.c -lusb

This tells the compiler to link your program with the libusb library. Make sure that you have installed the libusb library on your system and that it is in the default search path for libraries.