Hi @bahman_shahrzad_asr you can install Community Document Server by using apps installer of nextcloud on the web page ( check the description of this app for more detail about it :
or install ONLYOFFICE Docs Community Edition ( server ) check this link: Installation Guides - Docs - ONLYOFFICE
for my part, I have installed the server of onlyoffice Docs Community Edition
after installing the apps or server you need to install Onlyoffice app with the apps menu :
if you have choose the Community Document Server apps according to the github of the apps :
" The community documentserver will automatically configure itself if no other document server is configured in the onlyoffice settings (“Document Editing Service address” is empty). All other “Server settings” should be left empty. "