I couldn't figure out what was wrong with the scripts I and GUI I just spent around 3 hours on, so I came here to see if you guys knew! I'll check the answers in the morning, so sorry if I can't reply.
Local Script:
script.Parent.Activated:Connect(function()
script.Parent.Text = "Sending Information..."
local Q1 = script.Parent.Parent.Q1.Text
local Q1A = script.Parent.Parent["Q1 Answer"].Text
local Q2 = script.Parent.Parent.Q2.Text
local Q2A = script.Parent.Parent["Q2 Answer"].Text
local Q3 = script.Parent.Parent.Q3.Text
local Q3A = script.Parent.Parent["Q3 Answer"].Text
local Event = game.ReplicatedStorage.TrelloSend
local Player = game.Players.LocalPlayer.Name
Event:FireServer(Player.Name,Q1,Q1A,Q2,Q2A,Q3,Q3A)
end)
Server Script:
local Event = game.ReplicatedStorage.TrelloSend
local API = require(game.ServerScriptService:WaitForChild("TrelloAPI"))
local BoardId = API:GetBoardID("Applications")
Event.OnServerEvent:Connect(function(Plr,Q1,Q1A,Q2,Q2A,Q3,Q3A)
local LR = API:GetListID("LR Application",BoardId)
API:AddCard("Application",Plr..": "..Q1.." "..Q1A.." "..Q2.." "..Q2A.." "..Q3.." "..Q3A,LR)
end)
Output:
22:08:56.087 - ServerScriptService.Proccesser:6: attempt to concatenate local 'Q1' (a nil value)
Thanks for reading, and possibly answering.

2 comments
By the way, the error was for line 6 of the server script.
For some reason, Q1 is nil, which is why you can't concat it (using ..). Check all args/answers for nil values and, if found, replace with a blank string before you concat.