ERR_NAME_NOT_RESOLVED

Hi I am trying to set up a subsite as a subdomain of another subsite and I am getting “This webpage is not available ERR_NAME_NOT_RESOLVED”

My primary domain is http://www.countymums.co.uk
My first subsite has been configured to use http://www.northhantsmum.co.uk

I now want another subsite to point to networking.northhantsmum.co.uk

Is this actually possible?

I have set up networking.northhantsmum.co.uk as a primary domain mapping.

Thanks

https://ww.wp.xz.cn/plugins/wordpress-mu-domain-mapping/