Solution
USRP RIO FPGA codes are based onto an architecture template that must be followed in order to work properly.
It means that you cannot simply create your own FPGA code and write it onto your USRP.
In order to add your own custom code, you need :
1. Open one of the existing USRP RIO Template
2. Read the documentation
3. Open the FPGA VI
4. Open either the Rx VI or Tx VI
5. Open RxCore VI (or TxCore VI)
6. Add your custom code between the FIFO and DDC VIs