FTDI d2xx driver not reading bytes on Android to communicate with MDB board to Vending Machine
I'm wondering where you searched for the return values, because if I type "Android D2XX FTDI" into Google it immediately and firstly returns to me this PDF.

Which very simply tells you the return values to check for in each function. In this case (page 47) it's the number of bytes written, not a status.

So it seems you are not sending just 2 bytes and not receiving any data does not have to be in your FTDI system, but can easily be in whatever you connected. Which you do not talk about. If you expect to send 2 bytes and send 5 bytes, it's possible you're also not sending the bytes you think you send.

You also say you tried different Baud rates. What does the hardware connected think of this? Did you reconfigure that? With UARTs, devices need to agree to the communication speed in detail.

It's also possible something else is going wrong in your connection that you forgot to check for, but that I cannot say, since I avoid Java like the plague. (Call it a personal preference)

On page 30 you can also see how to build up checks to prevent any untraceable errors.