Trouble adding an add-on domain

Hi there! I am trying to add a new website domain to my hosting, but keep getting the error:

Quote:(XID 3trcdt) Could not determine the nameserver IP addresses for “meganlee.com.au”. Please make sure that the domain is registered with a valid domain registrar.

Can you please help me?

I have checked that the DNS servers are set up correctly at Namecheap, and this is not my first addon domain (the first one I added a few months ago worked fine).