Domain Mapping Invalid Status

Hi,

I have installed Domain Mapping Plugin for my multisite.

I tried to map one of my multisite sub-domain (http://apple.55stores.com) into my existing domain that I already have (http://wordpresswebsite.club). But the health status for the domain is 'Invalid', which mean I can't map the sub-domain.

May how I can get the Health Status to read VALID so that I can map it to the domain.

I would really appreciate some help with this.

Many thanks in advance.

Regards,
Daniel