www.pets.styleartc.com would be a second-level subdomain so is not covered by the Cloudflare Universal SSL certificate, which in your case only covers styleartc.com and *.styleartc.com.
You would need to use the Advanced Certificate Manager if you need an SSL certificate for that hostname. Alternatively you can use it as “DNS only”. In most cases, adding www as a second-level or deeper subdomain isn’t worth doing.