Sorry for your loss.
Can you find out what it was eating before you bought it? Did you see it eat before you bought it? Was it eating for you before copper? What are you treating it for with copper? Is it more important to treat that or eat?
Tempt your yellow tang with different foods.
If you aren't at recommended copper levels, then you might be doing nothing but stressing fish. Have heard must be at recommended copper levels. Have also heard tangs do poorly to medications. Have heard yellow tangs are to reef tanks what canaries are to coal mines. My experience supports that: my yellow tang nearly died during a Reef Flux treatment so went to QT 3week vacation - all other tangs, fish, inverts & corals were fine during display tank treatment. Had to use acclimation box to reintroduce from QT back home to main display tank at end of treatment (sailfin wanted to kill yellow (again)). My sailfin and yellow always fought daily and sailfin was recently rehomed (due to CBB and not due to yellow). Suspect the similar sizes and similar nose shape contributed.
My yellow tang was slow to take to nori sheets and Reef Frenzy, but tore up Red Ogo from
@AlgaeBarn and live black worms - but mine came from the wild right end of last year before Hawaii ban so those are things it was probably already familiar or similiar. I'm currently growing live red ogo in a 5g PetSmart tank with airstone, dosing bit of nitrate, bit of phosphate and drops of chaeto grow each week - has a 5k LED lightbulb sitting over its eggcrate top. It L.O.V.E.S red ogo and now will eat nori
I always feel fish in QT needs a friend. My go-to fish for QT friend is a neon blue goby because they are small - my QT is small - and they have lots of personality and don't get lost in my 180g once both fish out of QT (and I can have multiple blue gobies; zero aggression).
When it comes to picky eaters - try everything and try it repeatedly. These were left overs from a recent fish introduction:
