[ Solved ] - Unable to create new child domain

Here I’m getting this error message while creating a new child domain:

‘Error message: Invalid domain.’

I’m non-technical, please help me to understand!

Thanks

Sorry, I made a silly mistake actually, I was not adding the complete domain under the domain name bar as it should be ‘subdomain.example.com’, I was adding the only name of the domain, such as ‘subdomain’ only.