I was able to solve my issue thanks to your last message.
It turns out, the USB peripheral clock need to be enabled to access the USB RAM.
It makes sense since the USB RAM is tightly coupled to the USB peripheral but I haven’t found a clear mention of that in the user manual.
Anyways, here’s the line of code that solved my problem:
CLOCK_EnableUsbfs0DeviceClock(kCLOCK_UsbfsSrcFro, CLOCK_GetFroHfFreq());
Thanks again!