Cannot press the “Back Up This iPhone” button

See -> If you can't back up to iCloud -> If you can't back up to iCloud - Apple Support

👉 If the iCloud Backup option is greyed out

1. Check if a restore is still in progress. Go to Settings > [your name] > iCloud > iCloud Backup. If your device is still being restored, you see “This [device] is currently being restored and will automatically back up when it is done.” You can create a new backup after your device finishes restoring.
2. Check whether you installed a profile that restricts iCloud Backup. Go to Settings > General > VPN & Device Management to see a list of installed profiles. 

See this Apple article: How to back up your iPhone, iPad, and iPod touch

How to back up your iPhone, iPad, and iPod touch - Apple Support

Try touching the "Learn more" highlighted text.

👍 yes, try again later after backing up your iPad.

Still not work.. how to create a new backup ?

Still not work.. how to create a new backup ?