We can change the WhatsApp number, Call to Action, prefilled message while editing the post from the right side bar’ )
Change Settings at Page Level
Add WhatsApp Number with Country Code
If already added it might be cached version, Clear the Cache and try
If still an issue, please contact us
Click to Chat -> Other-Settings -> Group, Share features -> Enable Group Features/ Enable Share Features
Chat settings are enabled by default. If like to hide chat on all pages select show on selected pages.