I am unable to register any mobile devices to my account

Hello davide8,
 
Welcome to the Webroot Community,
 
Is Webroot asking for the invitation code as this is for the Webroot Business version only and the invitation code requires an Apple Certificate from the Apple site.So please uninstall the Webroot Business version.
 
And at this time, there is not an antivirus option available for Apple iOS devices. Because of how Apple manages the apps available in their App Store, there is not a need for one currently. This may change in the future but at this time is is extremly hard/almost impossible to infect an iPad/iPhone
 
If so, it's important to know that your iOS device will not show up under "mobile protection" in your account, regardless of what version of Webroot you have. SecureWeb and SecureSync are Webroot apps for iOS devices, but they are not anti-virus applications and thus will not appear in the mobile anti-virus portion of the account area.
 
Webroot does offer two apps you can download from the App Store to backup your files and protect your internet browsing, called SecureWeb and Backup & Sync. You can find either of them by searching for "Webroot" in the App Store on your Apple iOS device. For more information on both of these products please use the information below.
Webroot SecureWeb
https://mobile.webroot.com/brands/webroot/en/help/ios/SecureWeb/index.htm
Webroot Backup & Sync (for SecureAnywhere Complete subscriptions only)
https://mobile.webroot.com/brands/webroot/en/help/ios/SecureSync/index.htm
 
 
But here is more in depth information for WSA Complete for IOS,
First, let's make sure you're following these instructions for installing Webroot SecureAnywhere Internet  Security Plus on an iPad:
 
iOS Download Instructions 
To download the SecureWeb app:

1. If you have not yet created a Webroot account, go to https://my.webrootanywhere.com. Click Sign up now to begin creating the account.  
2. Open the App Store on your device and search for “Webroot SecureWeb.” Tap to install the app. Alternately, you can download and install the SecureWeb app from iTunes at: http://itunes.apple.com/us/app/webroot-secureweb-browser/id460986853?mt=8  
3. Enter your Apple ID password. The SecureWeb app opens and installs on your device. Once installed, you will see the Webroot SecureWeb icon on your Home panel. Tap this icon to open the app.  
4. To access Password Management functions, Look here at the Mobile IOS User Guide  
5. For more information, see Webroot SecureWeb for iOS Help.

You will need tpo check your permmissions  to your account permissions, sign into http://my.webrootanywhere.com, click your email address at the top right of the console, click Manage Users, and click the little person with the pencil icon on the right side of the screen.  Ensure that all of the values for your user are set to either Access or Admin level and that there are checks in the Passwords and Passwords Services boxes.  If anything is not set to that level and cannot be changed or a box is not checked and the value cannot be changed, please let us know.