[Seerah]

You need to wait until these frames are setup before you can hide anything. Try this:

local f = CreateFrame("Frame")    --we need a frame to listen for an event
f:RegisterEvent("PLAYER_ENTERING_WORLD")   --we'll listen for this event - all the UI stuff should be done loading by now
f:SetScript("OnEvent", function(self, event)   --when the event fires we want this to happen
     PlayerFrameTexture:Hide()
     PetFrameManaBar:Hide()
end)

Put the above into your Lua file, or you can use this to make your addon instead: https://addon.bool.no/