Why does it return an error when I add a sitemap file in Google Webmaster Tools?

• You can always utilise one of the various sitemap generators and plugins available. You risk facing a lot of issues if your document is not correctly structured and uploaded.
• Go to the homepage of your Webmaster Tools account.
• Go to the webpage by clicking on it.
• Under Crawl, select Sitemaps.
• Choose the sitemaps you want to resubmit.
• To finish the process, click the Resubmit option.
• If the sitemap you uploaded is empty, you're likely to see this error. Because the Google bots cannot recognise the information in the format, improper tagging of URLs in the sitemap may also result in this issue.
• Start by double-checking the sitemap to make sure it isn't empty. Make sure the URLs are appropriately labelled. This information may be found in the sitemaps guidelines, which you must read and comprehend before even considering building sitemaps.

Read More: What must I index all my pages on Google Webmaster, or let it auto-index?