Hello,
http://prntscr.com/l7ir1d
Please make sure you are uploading only the installable theme which is called XStore_Theme_v.5.1.zip
https://help.market.envato.com/hc/en-us/articles/202821510-Theme-is-missing-the-style-css-stylesheet-error
https://www.8theme.com/documentation/xstore/theme-installation/theme-update/
Regards