Hello there and welcome to the community!
This error happens when something is wrong with your blocks, so it would be useful if you can share the ones causing the issue (if you know which one they are).
First of all i suggest you to read here on How to ask Great Questions v2.0 - Learn About the Thunkable Community - Community.
For the troubleshooting procedure, i suggest to try your app block after block, when the error occurs it means the last block you added is where the error is.