I am trying to enroll my iPad, and it wants an Invitation code. What is it?

Hello there, welcome to the Webroot Community!
 
Let me state up front, I am not a user of Apple products, though I do have access to some good material here on the Community that may help.  Perhaps ? will also take a look here as she knows iOS far better than I.  She is the source of the below information.
 
 
 
The invite code is, I think, required only for Business Endpoint versions of WSA.  If you are needing to activate your own personal license, and not one provided to you by your employer, the business versions of WSA will not be the ones you want.
 
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.
 
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, tap the key icon in the SecureWeb toolbar. Enter your SecureAnywhere user name and password. This is the same login that you use to access your Webroot account at my.webrootanywhere.com. If you have not yet created a Webroot account, go to my.webrootanywhere.com. Click Sign up now and complete your registration. When SecureWeb validates your Webroot credentials, a green checkmark appears above the key icon.  
5. For more information, see Webroot SecureWeb for iOS Help.

You will need to 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.
 
Hope this helps?