Hello,
Please point me to the guide that you used for the installation. Also please let us know version of installed Document server.
Please bear in the mind that official installation guides are placed on our web site: Installation Guides - Docs Community Edition - ONLYOFFICE
If you have installed the Nextcloud forked version of Document server, please remove it and use the guides from the link above. The information about forked version is here: Community Document Server app in Nextcloud 18
If you have installed the correct version of Document server, please let us know the connector app version, the Document server version, reproduce the situation with the integration (click Save button on the connector app page) and provide us with whole Document server logs folder. It’s located here:
docker: /app/onlyoffice/DocumentServer/logs/documentserver/
deb\rpm: /var/log/onlyoffice/documentserver/
Windows: Program Files\ONLYOFFICE\DocumentServer\log