HTML5 package after the operation, there is an error, consider migrating to WebAssembly for optimized memory usage

The error is actually something else, but here it looks like the call stack is so long that the error does not show through at the very top of the page. Try right-clicking on the page, then choose “Select All” and then “Copy” and then Paste all of that text to a text editor to see the whole message, especially in the beginning.

There is a check box in the project settings to enable WebAssembly. Also try updating to latest UE 4.17 to see if that might have fixed the error.