[Fizzlemizz]

Your code above, the target frame (ft) is assigned an OnEvent script using:

ft:SetScript("OnEvent", function(self) SetPortraitTexture(self.Texture, "target")  end)

It didn't ever assign the OnEvent function (which would have required using a :HookScript or else it would have only replaced (overwritten) the original script that set the portrait texture if you had used SetScript).

This is an example using a single event frame (EventFrame) for all units rather than an OnEvent script for each unit frame. The separate event frame is not required eg. you could use the Player frame for all events, it just helps to make clearer what is happening.

You can add events to OnEventFunc and use it to update the units needed for that event.

--Target Classification
local default = "Interface/AddOns/ClassicPortrait/media/default"
local uClassTextures = {
    worldboss = "Interface/AddOns/ClassicPortrait/media/worldboss",
    boss = "Interface/AddOns/ClassicPortrait/media/boss",
    elite = "Interface/AddOns/ClassicPortrait/media/elite",
    rare = "Interface/AddOns/ClassicPortrait/media/rare",
    rareelite = "Interface/AddOns/ClassicPortrait/media/rareelite",
}
 
local UnitFrames = {} -- holder for unit frames created, Stored as UnitFrames[unit id] = frame
 
local fp = CreateFrame("Button", "FarmootPlayerPortrait", UIParent, "SecureUnitButtonTemplate")
UnitFrames.player = fp -- add the player frame to the UnitFrames table
fp:SetSize(70, 70)
fp:SetPoint("CENTER", -80, -350)
fp:SetAttribute("unit", "player")
RegisterUnitWatch(fp)
fp:SetAttribute("toggleForVehicle", true)
fp:RegisterForClicks("AnyUp")
fp:SetAttribute("*type1", "target")
fp:SetAttribute("*type2", "togglemenu")
fp:SetAttribute("*type3", "assist")
fp.Texture = fp:CreateTexture("$parent_Texture", "BACKGROUND")
fp.Texture:SetAllPoints()
--SetPortraitTexture(fp.Texture, "player")
fp.Border = fp:CreateTexture("$parent_Border", "BORDER")
fp.Border:SetPoint("TOPLEFT", -6, 4)
fp.Border:SetPoint("BOTTOMRIGHT", 6, -10)
fp.Border:SetTexture("Interface/PLAYERFRAME/UI-PlayerFrame-Deathknight-Ring")
fp.Border:SetVertexColor(1, 1, 0, 1)
 
--Target Frame
local ft = CreateFrame("Button", "FarmootTargetPortrait", UIParent, "SecureUnitButtonTemplate")
UnitFrames.target = ft -- add the target frame to the UnitFrames table
ft:SetSize(70, 70)
ft:SetPoint("CENTER", 110, -350)
ft:SetAttribute("unit", "target")
RegisterUnitWatch(ft)
ft:SetAttribute("toggleForVehicle", true)
ft:RegisterForClicks("AnyUp")
ft:SetAttribute("*type1", "target")
ft:SetAttribute("*type2", "togglemenu")
ft:SetAttribute("*type3", "assist")
ft.Texture = ft:CreateTexture("$parent_Texture", "BACKGROUND")
ft.Texture:SetAllPoints()
ft.Border = ft:CreateTexture("$parent_Border", "BORDER")
ft.Border:SetPoint("TOPLEFT", -6, 4)
ft.Border:SetPoint("BOTTOMRIGHT", 6, -10)
ft.Border:SetVertexColor(1, 1, 0, 1)
 
local function OnEventFunc(self, event, ...)
    local arg1 = ... -- add more args depending returns from on events used. UNIT_xxx events return the unit id ("player", "target" etc.) as arg1 (being first parameter of ...).
    -- UnitFrames[arg1] eg. UnitFrames["player"]
    if event == "UNIT_PORTRAIT_UPDATE" and UnitFrames[arg1] then
        SetPortraitTexture(UnitFrames[arg1].Texture, arg1)
    elseif event == "PLAYER_TARGET_CHANGED" and UnitExists("target") then
        SetPortraitTexture(UnitFrames.target.Texture, "target")
        UnitFrames.target.Border:SetTexture(uClassTextures[UnitClassification("target")] or default)
    end
end
local EventFrame = CreateFrame("Frame")
EventFrame:SetScript("OnEvent", OnEventFunc)
EventFrame:RegisterEvent("UNIT_PORTRAIT_UPDATE")
EventFrame:RegisterEvent("PLAYER_TARGET_CHANGED")