[Plugin: Google XML Sitemaps] Error While Rebuilding sitemap

hi.
i cant rebuild my site map manually. when i click on rebuild site map this error appears :
Fatal error: Allowed memory size of 67108864 bytes exhausted (tried to allocate 72 bytes) in /home/winparsc/public_html/wp-includes/load.php on line 552
plz tell me why ???

https://www.ads-software.com/extend/plugins/google-sitemap-generator/