Hi Sally,
I’m sorry to hear you are experiencing difficulties.
The first thing I want to note is that though ideally we’ll be able to help and properly register the license, the plugin’s general functionality will not be impacted if you are unable to do this. The only real hitch, then, is that you will have to update manually rather than benefitting from automated updates.
To the problem at hand: I’m unsure why you are getting this error message but there are a few possibilities. One is that your theme, another plugin or some piece of customization work is getting in the way.
I’d like to eliminate those possibilities from our enquiries first of all, so can I ask you to run through our standard troubleshooting processes in the first instance?
Thanks!