The reason this happens is because Thunderbird (as of version 128.3) doesn't support RSS autodiscovery, so if you give it a URL of a webpage (e.g. a website's homepage), it won't know what to do with it. Thunderbird needs the URL of the RSS feed, not of the webpage.
Typically this URL ends in /feed/ for WordPress websites, or /rss, or .rss.
See [How to Find the RSS Feed URL of a Website] for details.
See also [How to Add an RSS Feed to Thunderbird].
Observations
Thunderbird shows a link at the right of this message that reads "Check validation and retrieve a valid url." This sounds like Thunderbird has the functionality to validate and figure out where the RSS feed is from a webpage, but it does not.
The link opens a new tab inside Thunderbird that goes to W3's validator [example using blog.thunderbird.net], but the validator displays the message "It looks like this is a web page, not a feed. I looked for a feed associated with this page, but couldn't find one. Please enter the address of your feed to validate."
This means that this link to W3's validator is worthless.
Thunderbird's blog was made with WordPress, and everybody uses WordPress. WordPress declares where the RSS feed URLs are in the HTML code of its pages using the standard RSS Autodiscovery method. Vivaldi can find them in Thunderbird's blog just fine. In other words, websites made with WordPress wouldn't have their RSS feeds findable by this tool, and this means a lot of websites.
In fact, I'm not sure what kind of feeds it can find, if it can find anything at all. From the looks of it, it's only meant to validate URLs that actually go to the RSS XML files, and not supposed to find anything their URLs from HTML. I'm not sure why this link was added. I assume when it was added, W3's tool was actually able to find feeds' URLs, but maybe it was updated without Thunderbird's knowledge, and now it's just not meant for this purpose any longer.
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