DeveloperConsoleVisible should be DevConsoleVisible, trying to use :SetCore or :GetCore with DeveloperConsoleVisible will error
https://developer.roblox.com/en-us/api-reference/function/StarterGui/SetCore
game.StarterGui:SetCore("DeveloperConsoleVisible", (not game.StarterGui:GetCore("DeveloperConsoleVisible"))) --Errors
game.StarterGui:SetCore("DevConsoleVisible", (not game.StarterGui:GetCore("DevConsoleVisible"))) --Doesn't error