line 747 causing lua errors, the function clickedWithPeddlerCombo() does not exist in Classic Era version Peddler.lua
Any workarounds (except adding it?)

I simply added this function before the line(s) in question:
local function clickedWithPeddlerCombo() return false end
Simply return false to avoid the error but quest rewards cannot be flagged in the quest frame.

Thanks @ElegantSerr, no more errors now!

Adding more details to this, since it's a permanent issue on Classic Era - when you select an item quest reward, Peddler throws this lua error:

Message: Interface/AddOns/Peddler/Peddler.lua:739: attempt to call global 'clickedWithPeddlerCombo' (a nil value)
Time: Sun Dec 10 13:00:45 2023
Count: 2
Stack: Interface/AddOns/Peddler/Peddler.lua:739: attempt to call global 'clickedWithPeddlerCombo' (a nil value)
[string "=[C]"]: in function `clickedWithPeddlerCombo'
[string "@Interface/AddOns/Peddler/Peddler.lua"]:739: in function <Interface/AddOns/Peddler/Peddler.lua:738>
[string "=[C]"]: ?

Locals: (*temporary) = <function> defined =[C]:-1