In this case something went wrong at an earlier stage of the installation. Usually you should find several different tables in the Nextcloud database. You should try to install Nextcloud from scratch, as described in the documentation.
Unfortunately my French is not so good to answer, but to read the articles. It seems that it might be a little bit tricky to install Nextcloud on an OVH web server. I’ve collected some additional information which might help you with your installation. Eventually @Vincent_Bardet can provide some support because he managed to install the software successfully:
Thanks j-ed. Unfortunately nowhere in your links is my problem solved or even addressed. In my case, the installation script doesn’t create a single table in the DB.
If I willingly provide wrong DB credentials, I get another error: Error while trying to create admin user: Failed to connect to the database: An exception occured in driver: SQLSTATE[HY000]  ProxySQL Error: Access denied for user 'whatever'@'[redacted ip address]' (using password: YES)
So we can consider in the OP’s problem that the credentials are right. it’s what happens next that has problems.
The SQL error says that you’ve configured Nextcloud to access the database, or better to say MySQL proxy, using the mentioned IP address. You have to make sure that your database excepts connections on that interface for the mentioned user. Here you will find information how you can check the user accesses: