This is my code. I have no idea why it ain't working.
local main = script.Parent.Parent
script.Parent.MouseButton1Down:Connect(function()
local http = game:GetService("HttpService")
local rank = main.First.Rank.Text
local username
local priority = tostring(main.Priority.Value)
local number = tostring(main.Second.Num.Text)
local reason = tostring(main.Second.Rea.Text)
local Data = {
username..", "..rank.." has sent a priority "..priority.." back-up request. "..number.." personnel members are requested for the following reason: "..reason;
}
Data = http:JSONEncode(Data)
http:PostAsync("https://discordapp.com/api/webhooks/...", Data)
end)

3 comments
is this a local script? You must post requests on the server, not on the client. Use RemoteEvents for communication. Your [Data] also makes no sense, you need to send what discord requires for it's embed, in your case you need to specify [content] inside of the table, not just send a string.
Take a look here: https://discord.com/developers/docs/resources/webhook, scroll down to [Execute] method, it shows exactly what discord webhook requires.
@imKirda No, it is a serverscript. I still cannot figure out how that works. Would you help me?