[Fizzlemizz]

You're addon is probably running the code before Bartender has initialised the bars. This usually happens at a specific event that fires during the game loading sequence.

It's also why it works when you're already in the game tesing in WowLua.

Try something like:

function ParseSpellName(spellId)
   local spellName, spellSubName, _ = GetSpellInfo(spellId);
   if spellSubName then
      return string.format("%s(%s)", spellName, spellSubName)
   else
      return spellName
   end
end
 
 
local f = CreateFrame("Frame")
f:RegisterEvent("PLAYER_ENTERING_WORLD")
f:SetScript("OnEvent", function(self, event, ...)
    self:UnregisterAllEvents() -- PLAYER_ENTERING_WORLD will fire every time you see a loading screen so, stop that happening
    local button = BT4Button73
    local slot = button:GetAttribute('action');
    local actionType, actionId, _ = GetActionInfo(slot);
    if (actionType == 'spell') then
       local command = string.format("/use [@mouseover]%s", ParseSpellName(actionId))
       print(command);
    end
end)