The current stable distribution of Debian is version 9, code named stretch. It was initially released as version 9 on June 17th, 2017 and its latest update, version 9.4, was released on March 10th, 2018. Do not try to install DirectAdmin version cleanly in any way on Debian 9.4. Because Direct Admin currently does not recognize this version.
This issue has been solved by DirectAdmin support.