Hello,
in AI2 the lists start by 1 (instead of 0, like in other languages). That error indicated that you are trying to access with a index=0.
If you share your relevant blocks maybe any user can help you.
When you are testing your app (with Companion or emulator) you can use DoIt feature to debug your app (or use labels to display intermediates values/results).
Here you have other tips relative to debugging:
FAQ Section: Debugging - Frequently Asked Questions - MIT App Inventor Community