Works in Studio, not Client?

You had a mix of if's and else's in there, so I cleaned it all up..(Might have been the problem) Plus I made the AddCard Event a variable!

Script:

ap = require(script.Parent.Parent.TrelloAPI)  
local Board = ap:GetBoardID("Admittance")
local List = ap:GetListID("Admitted",Board)
local debounce = false -- just so they dont spam click the submit button

script.Parent.MouseButton1Click:connect(function() -- button connection
    if debounce == false then -- click filter
    debounce = true
    local check = game.Players:FindFirstChild(script.Parent.Parent.Patient.Text)
    if check == nil then -- checks if nil
        script.Parent.Text = "Please enter a valid name"
        wait(2)
        script.Parent.Text = "Send report"
    elseif script.Parent.Parent.Reason.Text == "" then
            script.Parent.Text = "Please enter a valid reason"
            wait(2)
            script.Parent.Text = "Send report"
    else
            script.Parent.Text = "Submitted"
            wait(5)
            script.Parent.Parent.Parent.Admit.Visible = false
            script.Parent.Parent.Parent.Button.Visible = true
            local card = ap:AddCard(script.Parent.Parent.Patient.Text,script.Parent.Parent.Reason.Text,List,"","top")    
    end
    debounce = false
    end
end)

Hope this works!