Cannot set up CDN via WP AWS Plugin

I have tried everything my little bit of knowledge of AWS etc to install Cloudfront on my WordPress Lightsail Instance. No matter what I try it fails.

My latest attempt is via the AWS WP plugin:
I can get all the way to
Check Status of Cloudfront DNS Records
Then I get varying messages – I was using https://www.example.com but that already exists as an A DNS record pointing to the Static IP so I have tried using gallery.example.com this time and I get this message:
Caught exception in method AmazonAI_Cloudformation::validate_cdn_alias_mapping in class AmazonAI_Cloudformation:
The CNAME or alias you entered (gallery.thestripeyhorse.com) is currently mapped to
If I check the Route 53 DNS records there are only the two records I have added as part of the set up.
I really have spent the last 2 days trying to get cloudfront installed and failed everytime.
I am hoping someone has an idea where I am going wrong please.

Thanks
Helen