The reason for this is that the radio is interrogating the car's body computer and is finding a different code than the one it expects. Some folks have surmised that it may be necessary to reprogram the firmware in either the radio or the body computer or even both, but I don't know of anyone who's done this successfully.
This has come up before, but I'm not aware of a solution. It may even not be possible to reuse the secondhand radio you have purchased in your vehicle.
Have a look at
this thread, and also try searching the internet. You'll likely come across several accounts of folks who can't get a secondhand FIAT radio they've purchased to work, but I've yet to see any from folks who've succeeded.
Sorry if this comes across as unhelpful, but it seems that swapping in a secondhand Fiat head unit from another vehicle is by no means straightforward.
All the advice I've seen is to just fit a generic aftermarket head unit that has the functionality you want.
If you are able to do this, it might save you much frustration.