Hello,
Thank you for contacting us and for using XStore.
1/ Please set an administrator role for the user you created for us.
2/ To add Google Tag Manager (GTM) code, you can insert it into the header or footer of your site. This is often done through the 3rd party plugin https://wordpress.org/plugins/wp-headers-and-footers/ , or by editing the header.php file (copy file from the XStore parent theme to the child).
Warm Regards,
The 8Theme Team