Okay, so they misspell "suppress" as "supress". Changing that fixed the UI error, but didn't suppress my alert.
I think I need to do something involving the file alerthandlers.lua, and I think I even found the right alert to suppress, but I'm not sure how exactly to suppress it.
Edit:
This works!
Code:
local handlers = ZO_AlertText_GetHandlers()
handlers[EVENT_OUTFIT_EQUIP_RESPONSE] = function() end