Note: Make sure to use latest v3.3 version as it contains many related fixes.
It is usually an error occurred while app compiling fails due to some reasons, it's not a major issue, you can follow simple steps below to get rid of such an error:
1) You have installed Java 7 or Java 8. (Java 9 or 10 not supported yet.)
2) You didn't used any unsupported special characters in app name, about text or any other info.
(accented filenames are not supported), Single Quotation mark is also not supported in App Name or About Text in v3.1. v3.2 supports most of them.
3) If using 'Local HTML' option, don't use unsupported filenames. Optimize all images in local HTML folder.
4) Your PNG's are optimized and Real PNG's(Make sure that you didn't renamed to png from jpg, jpeg, etc.). For confirmation, you can try a build without changing icon and splash, if succeeds, surely it's PNG error.
5) You may Re-Install Website 2 APK Builder. (Some files might have been corrupted).
6) If there are multiple installations of Java, remove all versions of Java and install again.