Hello @lisaleague, it's not Opera that needs to add support for Canva but the other way around :). Feel free to ask them about it, and make sure to let them know you use Opera, a browser based on Chromium, which should be perfectly able to show the site.
If you want to use the site in Opera anyway while they keep doing their browser detection, try using a user agent switcher from the addons store at https://addons.opera.com/en-gb/ (this allows the browser to pretend it's chrome).