Could you share your domain name here with us so we could double-check, troubleshoot and provide some feedback information related to your issue?
Firstly, you might be missing the www hostname at the DNS tab of Cloudflare dashboard. If so, kindly see the steps from below article and add the missing www record as follows.
Otherwise, may I ask if you have used some kind of a 3rd-party service like Kajabi, Shopify, eZoic or some kind of a hosting partner like WPEngine, or had a partial/full CNAME setup for your domain name, or even used your domain or website as a SaaS? Then, below article and tool might help you in this case:
The error I get shown on the screenshot from below is familiar with Shopify issue, kindly use the 2nd article and the tool mentioned inside it to resolve your naked domain and www hostname with it, if so.