Keil Uvision Error after choosing USB communication

Hi,

I'm using a STM32L100RC discovery board for a project. I'm trying to communicate with the USB port (Communication Device Class). After configuring the USB and USB_DEVICE part with STM32CubeMX, I have the following errors on Keil : 

".\Objects\Keil_TEST.axf: Error: L6218E: Undefined symbol MX_USB_DEVICE_Init (referred from main.o).
.\Objects\Keil_TEST.axf: Error: L6218E: Undefined symbol hpcd_USB_FS (referred from stm32l1xx_it.o).
Not enough information to list image symbols.
Not enough information to list load addresses in the image map.
Finished: 2 information, 0 warning and 2 error messages."

I tried to search for more information but I can't find anything. Does anyone know how to solve this issue? 

Thank you in advance,

Jay.