@GlenShennan - An organizational account isn't necessary, however, Community, Trial, Evaluation, and Personal use subscriptions are not allowed to create API keys. If you were signing in using one of those account types, you would receive an error. The user experience for these subscription types will be enhanced in a later update to the site.
To get around this, you would need to create a new free developer account: https://developers.arcgis.com/sign-up/ or sign in to an organization account.
Glad you were able to get around the issue!
Lauren