Domain Mapping Invalid Status


I have installed Domain Mapping Plugin for my multisite.

I tried to map one of my multisite sub-domain ( into my existing domain that I already have ( 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.