You should search for similar questions and tutorials on DevForum before you make a new post. Here are some I found from a quick google.
[TUTORIAL] How to add “Redeemable Codes” in your game? - Bulletin Board - Developer Forum | Roblox
How to make a Code GUI (for beginners) - Resources / Community Tutorials - Developer Forum | Roblox
How would I make a code system - Help and Feedback / Scripting Support - Developer Forum | Roblox