Give a little more detail about what you are working with.
Is this a BRM swapping into an original TDI car that had a BHW? Or was it a gasser being swapped to diesel? What harness and ECU are you running? ECU tune?
You won't need to swap the cluster (won't be able to anyway from an A5 to B5.5) provided you have the ECU tuned for an immobilizer delete. You have done that presumably?
Do you have OBD communication up and running yet? If so what codes do you see? If it's not starting due to a hard fault somewhere, this will help you find it.
Did it ever run at any point out of the donor car, for example did you get it started as a standalone setup but it won't run now that you have it in the car? Or is this the first attempt?
See cranking RPM in live data when turning over on the starter?
There are a million reasons why it might crank but not start, ranging from simple (something unplugged, ECU not getting power, etc) to complex (swap components not compatible, etc). We need a little more info in order to be able to help you narrow it down.