My discord webhook is not working. Why is this happening ?

well your code doesn't seem to be in a straight hierarchy. on line 6 you did http:JSONEncode()withHookData, but HookDatais defined on line 8 aslocal.. so will have to either move HookData = http:JSONEncode(HookData)` below where the HookData Object on line 8 is defined

or

define the HookData object as global, this way so its defined first(Global variables are defined first, if I remember right)

EDIT

ALSO, HTTP can only send numbers, and strings(text), and nothing else. That's why you need to call http:JSONEncode() to turn the table into a JSON-like text, which will be parsed by the server your sending it too into an Object or Array, therefore, b/c HookData.Content is set to LPlayer- which is a roblox Object,it might cause an issue.

if you want HookData.Content to be player's name then you'll have do player.Name, or call the built in function called tostring() which returns the name of the roblox object it's passed.

also, HttpService doesn't work in LocalScripts..it only works server side,and keep in mind you can't do game.Players.LocalPlayer in a server-sided script.

here is your code revised:

local Lplayer = game.Players.LocalPlayer --can't do this in a server script.., find another way to access a player. for instance, by players[PlayerName], or Players.PlayerAdded, or remoteEvents/functions

local url = ""
local http = game:GetService("HttpService")

local HookData = {
 ['username'] = "Builderman",
 ['content'] = Lplayer
}
HookData = http:JSONEncode(HookData)
http:PostAsync(url, HookData)