You're having issues to sign up with your Apple ID? Don't worry, it's probably your iPad fault...
📶 Check your connection
First check your Internet connection. If you're reading this article, your connection should be ok 🙄. But please ensure it is sill good when signing in.
✅ Check your Apple ID
Sign into https://appleid.apple.com to ensure that your account is functional.
🔐 Activate two-factors authentication
To sign up with your Apple ID, you must activate two-factors authentication (2FA) in your iPad settings, as detailed here: https://support.apple.com/en-us/HT204915#setup
🔓 Sign Out from older devices
Disconnect older Apple devices that run iOS 9 or older: Sign Out of the Apple ID in these devices' Settings to be able to activate the two-factors authentication on newer devices, as explained on Apple's website.
🔍 Clear the camera
Temporarily remove any screen protector over your device's camera as this may interfere with Face ID authentication.
😓 Last resort
Uninstall the app
Open the iPad's Settings, then tap your name.
Tap Password & Security.
Tap Apps Using Apple ID.
Look for Fontself and remove it (swipe left)
Sorry but... 👇
Reinstall the app