WhatsApp backup stuck

It could be several reasons:

• Make sure that you have enough space for the backup ON BOTH iCloud and the device;
• Make sure you are on Wi-Fi;
• Make sure the phone is on a charger;
• Make sure it's not on battery saver mode.

If all of the above conditions are correct -- stop the backup, force restart the phone, start the backup again.

Try to sign out from iCloud > force restart > sign in to iCloud. Also the phone is not charging at the moment of the screenshot, try leaving it charging for 20-30 minutes during backup if you haven't tried it yet.

If you are doing backup for the fist time. It takes time. Do it overnight. Connect to WiFi to power. The next morning you are in.