After you finished the WhatsApp transfer from iOS to Android, if you are not able to use WhatsApp normally, for example, you have no option to back up WhatsApp to Google Drive, then it is probably because your android phone still has the special version of WhatsApp. In this case, please follow the video guide below to upgrade to the official version on Android phones:
For WhatsApp:https://drive.google.com/file/d/11POyHpZGebyCDuS08Y3nohdYGYLG07rS/view?usp=sharing
For WhatsApp Business:https://drive.google.com/file/d/12G9of9FAfFS047DXD-hq0ZognoJAfYhD/view?usp=sharing