I can’t restore my WhatsApp backup on iCloud, the download progress is stuck at 51%
Thanks for reaching out using Apple Support Communities. Here to help.
We understand you are concerned regarding the restore of your WhatsApp chat history.
The link below will provide general guidance regarding the restore process.
How to restore your chat history - WhatsApp
We also notice in the screenshot provided that there is a message about being connected to a power source as well as a Wi-Fi network. The download may have paused until those conditions are met. We would suggest finding a Wi-Fi connection and plugging into power to see if this helps resume the download.
Hope this helps! Take care for now.