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) Make sure you don't use any older version other than v3.4 or v4.0. Don't use any special characters in app title.
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.
Warning: If you updated to v4.0 on 6th May, you should re-download latest fixed version of program from the website to avoid the issue.