Make sure the software is up to date on all devices involved.
Update macOS on Mac - Apple Support
Update your iPhone or iPad - Apple Support
If you see a message saying to try again later, see if it's a system issue: Apple - Support - System Status
Sometimes it helps to have the devices involved take a fresh look at settings:
● If this is a service, try disabling it and turning it back on.
● Try signing out of accounts and back in again.
Sign out of iCloud on your iPhone, iPad, iPod touch, Apple TV, or Mac - Apple Support
Sign in with your Apple ID - Apple Support
If you have more than one Apple ID, don't forget you may be signed into other accounts too for special services such as iMessage and Media & Purchases.
● Restarting is a simple step that can resolve many unexpected behaviors.
iPhone: Restart your iPhone - Apple Support then choose your model.
iPad: Restart your iPad - Apple Support
Mac: Ways to start up your Mac - Apple Support