The Undertale Tower Defense script link you're referring to is likely a resource for a game developed by fans of the Undertale series. After some research, I found a few possibilities:

Here's a step-by-step guide to creating a basic tower defense game using the Undertale Tower Defense Script Link:

Because the original is discontinued, many players have moved to similar, newer Roblox games such as Undertale Timeline Corruption: TD Undertale Defenders ⚠️ Note on "Scripts" and Security

: Several new fan-made projects have emerged to fill the gap, such as Undertale Adventures Undertale Timeline Corruption Gameplay Mechanics