Can’t add phone number to iMessage on iPad I gave to my wife
Can’t add phone number to iMessage on iPad I gave to my wife (her number, not mine)
iPad Air 2, iPadOS 14
Presumably her number is an iPhone number, otherwise it cannot be added.
When you gave the iPad to your wife, this was required.
What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support
Then she had to “move in” to her brand new (software-wise) iPad using her own unique Apple ID and password. Then she should be able to set up iMessage and FaceTime.