Does your device actually have an 8MHz crystal connected to pins 12 and 13 on the MCU? Many of the ST-based development boards I've seen have the 32kHz crystal for the real time clock but rely on the internal oscillator for main system clock.
The schematic does show the 8MHz crystal but also says that it is socketed - you may need to add it yourself.
The alternative is to use the internal oscillator and the internal PLLs to get the correct frequency for the USB device. The clock tab in the STMCubeMX IDE will tell you when you have it right.
One other way to tell is to start with something simpler such as the flash a LED, or even try to debug a simple bit if code - if the code runs at all then the system clock settings are OK (although the frequency may still be wrong).
Susan