Your flash and DSP code are probably fine. The flash code is the radios operational software. If it was faulty it would most likely fail the power up checksum test and give an error or just lock up. The DSP code is the instructions for the DSP engine to do all the analog and digital audio processing. If it faults you would see a DSP error and have no speaker sounds or transmit modulation.
Second, do not panic when you see the display on the radio showing "app err" . They all show this all the time, it is not an error. A tip I learned a while back is to always let it finish its program cycle on its own without intervention. It usually is just fine.
It sounds like you entered the feature code wrong. Even with no features the radio will still operate in conventional (without provoice). Error 551 reads "Personality feature encryption read failure or data not available." You can copy the feature string from the email and using the right click of your mouse paste in the new feature string in radio maintenance.
After you load it again verify in the radio's feature menu that all your features are there. If they are not read the encryption back out of the radio and verify that it matches what TAC sent you. If it matches, simply reply back to TAC and explain what happened, they should be able to verify it and if necessary send you a new string.
If you kept a copy of the old features you can always load that back in for the time being.
FYI to all, the kind lady at TAC who does the feature encryption, Stephanie, is moving up to a new position and is only doing features part time now. Hopefully they will find someone else as talented as she.