you cant return it like this maybe you try this:

client side:

local playerId = nil

RegisterNetEvent(‘PlayerIdentifier’)
AddEventHandler(‘PlayerIdentifier’, function(id)
playerId = id
print(playerId)
end)

serverside:

RegisterNetEvent(‘GetPlayerIdentifier’)
AddEventHandler(‘GetPlayerIdentifier’, function()
local _src = source
local id = GetPlayerIdentifiers(_src)[3]
print(id)

TriggerClientEvent('PlayerIdentifier', _src, id)

end)