I'm not that familiar with headers but I thought the Last-Modified was optional and not available for all sites. It says seotoolsforexcel was last updated in 2016. Hmm.
How about you use the =WhoIsDomainUpdated("https://www.seotoolsforexcel.com") formula? Alternatively, you can use the Wayback Machine connector to check when a page was last indexed. Also you can check specific headers for any site with =HttpHeader(url,header)