Getting an error when trying to submit for sitemap

[neilp123]

Trying to run the site [ External links are visible to forum administrators only ] through the sitemap generator, but getting the following error:

There was an error while accessing the URL specified: [ External links are visible to forum administrators only ]
Please make sure to specify the correct website URL and resubmit your request.

I don't think there are any robots.txt file causing issues, but maybe I'm wrong?

Any help?

Thanks

[XML-Sitemaps Support]

Hello,

your homepage returns http code 500 (which means error), that's why it's not crawled although displayed in the browser: https://www.xml-sitemaps.com/http-headers-viewer.html?op=http-headers-viewer&go=1&pageurl=http%3A%2F%2Frspca-llysnini.org.uk%2F&se=xmlsitemaps&submit=View