The error NS#125 "Incompatible firmware version" is raised because the firmware in the device does not match the firmware in the application file.
The Series 6000 chip might already be loaded with firmware version 21. However, there have been several releases of firmware 21 and they are not compatible. If you have installed the latest Izot NodeBuilder, you need to follow the instructions from the Izot NodeBuilder readme file , section 5 "Updating from IzoT Nodebuilder 4.3" to update the EVBs to the latest firmware. You should also recompile any application to make sure it is using the latest firmware.
If this is a brand-new chip, it is loaded with firmware 19 and you will need to follow the instructions from FT 6050 first time programming (KB1347)