Tracking code is invalid
Is it? Is it really? Is there no possibility that the code is actually valid, but not in the system for some reason.
Tracking Code was not recognized
Not recognized? Did I get the case wrong? Why was it not recognized? Was it my fault or the computer's fault or the company's fault?
Neither of these messages really help me identify the problem in a way that helps me move forward. Neither of them give me the confidence to know whether the code is wrong or the system is not up to date.
So, I'd like to see a message that tells me why it's not working and a few tips on how I might fix it.
Tracking code needs to start with 'A' followed by 5 digits
Note that this is actually the sort of information that should be given to the user before entering the code so that they know they are using the right code.
If the code is the right format, but not found, then the message should take ownership of the problem, not blame it on the user or the code.
We can't find the code 33763 at this time.
Then depending on how the system works, give more information, e.g. if there's a possibility it might work if they come back later. A phone number to call, or some suggested next step rather than a flat refusal to help further.
Indicate somehow that the system is up and running and that it's not failing because the computer is off or something, take a small amount of time to appear to be doing a search. There is such a thing as too quick a response!