To allow other users to assist you, you should follow the usual rules and provided in detail information about which software versions you have installed, web server, php, database, Nextcloud etc.
This message usually indicates that a database connection couldn’t be established. Due to the fact that you didn’t provide any Nextcloud configuration details, I couldn’t give you any advise what need to be done here.
BTW, I assume that you’re using the Nextcloud app package from the Asustor?