Hello,
If you got that error it means you updated the XStore core plugin but did not update theme. Before every core plugin update, we would recommend you to check the changelog https://xstore.8theme.com/update-history/ and you’ll find that you need to update not just XStore core plugin but XStore theme first. Any core plugin update requires the theme update to the latest version before the plugin update.
So, update your theme to version 5.6 first https://xstore.helpscoutdocs.com/article/63-theme-update and after that proceed with the XStore core plugin update.
If you need our assistance, provide FTP access.
Regards