[admin@GW5ML] /ip dhcp-server network> add address=172.28.20.0/24 gateway=172.28.20.254/24 dns-server=172.28.20.254
invalid value for argument address
the error message could mean that an invalid value has been provided for address=172.28.20.0/24
in this case it is the gateway wich is incorrect
imho, a good error message would have been something like
invalid value for argument [argument name] ( [argument type] expected)
in this case
invalid value for argument gateway (address expected)