Anyone know the cause of this issue

That Suitelet is invalid, disabled, or no longer exists.

time to turn it off testing mode..? That's my first check anyway ^_^ make sure the deployment is active, and the script is live!

I tried deleting the deployment and recreating and that didn't work. So I deleted the script altogether and recreated and that worked. Its only a test SL so I wasn't worried about deleting important things

AFAIK if the script is in testing mode, it'll only run/work for that particular user that owns the script, i.e the script owner everyone else that runs might not have a valid deployment and run into an error in testing mode (testing mode instead of released in deployment)