Read the following tutorial https://codex.wordpress.org/Configuring_Wildcard_Subdomains
In Setting Menu > WooCommerce Category Subdomains > Choose to enable product or post categories > Save Changes
- This seems to be regarding cookies availability in subdomain issue. One of the solution is to change the cookie domain.
- Eg: go to wp-config file, add in define("COOKIE_DOMAIN", ".example.com"); need to add "." in front of the domain to support multiple subdomain.