Anytime you see the “HTTP 500: “Internal Server Error” message in Roblox, you’re either trying to publish your game or make changes to it on the backend.

What is a 500 internal server error? How to fix the common message you receive when a website won't load ...

Try to connect the device that has encountered the Roblox error code 266 to a different network to resolve the error code. Fix #5: Check for any internal server error.