You may get the error “Unable to write user’s httpd.conf” while creating a domain in DirectAdmin.

Unable to write user's httpd.conf
Could not include user's httpd file to the main httpd.conf

You are probably getting this error because you may have tried to delete this domain before, and there could be existing files of the domain under the directory “/var/spool/virtual”.
Delete these domain files under ( only this domain’s files ) this directory, and you will be able to create the domain again.