Method 1: Open Organizer in Elements 10,
Edit > Preferences > Adobe Partner Services. choose Canada/ USA for as Location. Hit OK.
Try this to see if you're able to use greeting card dialog.
If that doesn't solve your issue,
Method 2: Follow the steps outlined here: http://helpx.adobe.com/photoshop-elements/kb/error-valid-size-available-creation.html
Let us know how it goes.
I'm moving this thread to Photoshop Elements forum for better responses.
Yes, the article mentioned in Method 2 should serve the purpose. Do tell if you are able to resolve or not.