You can likely jump the chase if you just call Moog support and explain the situation but I can tell you (long time Tribute owner here), that assuming there is nothing physically wrong with your Phatty, the update procedure will vary.
Depends on a) what version Phatty is it? (has it been upgraded with additional memory, etc) b) what code did you have previously?
Somewhere along the line, and Moog can tell you for sure, they changed the portion of code that accepts new firmware so the procedure is somewhat different than you may have found online or in the printed manual.
Certainly if you do not have an upgraded Phatty, you cannot run version 2.1T or higher so that may very well be your issue. But there are very good pre-v2.x builds out there that add alot of features.
Some purists like their Tributes with v1.3 (or is it 1.11) code. Others want things like the arpeggiator, LFO reset upon kbd gate, pot mapping, etc.
Again, it’s been years and I’ve lost track of which build was which but Moog support should be able to guide you by first looking up your serial number (if you don’t know) to see if yours has been upgraded and secondly, to ascertain what the precise firmware procedure is and subsequently which version of code is best.
If you apply the incorrect firmware build, you might “BRICK” your Phatty, which will necessitate the utility of a SilLabs programmer to be physically attached to the digital portion of the board in order to bring it back to a healthy state. There is a YouTube video demonstrating this; it’s not the end of the world but to avoid the expense of this and your needing to open up the Phatty, best to first find out what you are dealing with.
Please update this thread with what you find out.
EDIT: here is a link to the old upgrade page (not sure if Moog is still offering this but worth asking if you are interested): http://legacy.moogmusic.com/littlephatty/?section=product&product_id=21300