Check the URL Format: Ensure that the URL you are entering is in the correct format. It should start with "http://" or "https://" and should not contain any spaces or special characters.
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Verify the RSS Feed Existence: Make sure that the URL you are using is indeed a valid and active RSS feed. You can try opening the URL in a web browser to see if the feed is accessible.
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Check for Typos: Carefully review the URL for any typos or errors. Even a small mistake can result in the "not a valid URL" issue.
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HTTP vs. HTTPS: Confirm that the URL is using the correct protocol (http or https). Some servers might only support one of these protocols.
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Firewall or Security Software: Check if any firewall or security software is blocking the connection to the RSS feed. Temporarily disable such software and try again.
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Server Issues: The server hosting the RSS feed might be experiencing issues. Try accessing the feed at a later time.
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Browser Cache: Clear your browser cache, as it might be storing outdated information about the URL.
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Redirects: If the URL redirects to another URL, make sure that the final destination is a valid RSS feed.
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Contact the Feed Provider: Reach out to the provider of the RSS feed and confirm that the URL you have is correct. They might have changed the feed URL or encountered issues on their end.
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Go to https://validator.w3.org/ input the RSS Feed link and then click Check

Once clicked Check, the website should give you information about the RSS Feed and whether it's working or not.