We built a new website with Vaadin Flow and added it to Google Search Console. We tried using fetch as google. But we get the following error:
I'm sorry, your browser is not supported
That implies that your website will not be letting Google crawl your pages.
Somewhere in your websites code will be a check for browser compatibility. And with that warning message. You want to find that and change things so that Googlebot is not considered an incompatible browser.
Googlebot is based of an old version of the Chrome browser. v41 ish. Maybe your website does not want people on old browsers to look at it?