Entry name 'classes.dex' collided
You are probably facing this error because you already have a signed APK in your project's "release" folder. Please check for any existing APK and delete it, then try to generate once more.
I had the same problem and here's how I solved it easily:
1. Build -> Clean Project
2. Build -> Generate Signed Bundle / APK..
Delete Your Old Signed APK from your App folder and then Generate Signed APK