In its current implementation, whenever you answer a video call on WhatsApp, the call defaults to your front camera. If you're, for any reason, not interested in showing yourself on camera, you ...

Fix #2: Reboot your Android phone. Another basic thing that you can do to fix unable-to-answer-calls issue is to refresh your device by restarting it.

WhatsApp beta version (2.25.7.3 for Android), as pointed out by Android Authority, reveals a countermeasure — one that will give you the option to answer a video call with your camera off.