Occ commands result in "Cannot write into "config" directory!"

Nextcloud version 14 :
Operating system and version Ubuntu 16 :
PHP version (7.2) :

Hello
I’m running different nextcloud instances on my plesk server and on all I am not able to make an occ command.

When I type:

sudo -u www-data /opt/plesk/php/7.2/bin/php /var/www/vhosts/nextcloud/occ -h

I get the answer:

Cannot write into “config” directory!
This can usually be fixed by giving the webserver write access to the config directory
See https://docs.nextcloud.com/server/14/go.php?to=admin-dir_permissions
Or, if you prefer to keep config.php file read only, set the option “config_is_read_only” to true in it.
See https://docs.nextcloud.com/server/14/go.php?to=admin-config

I’m running into this issue with all my 3 nextclououd installations.

I already searched the web for a solution but was not sucessfull. Where can I start?

Thank you in advance for all advices!
Lukas

You found a dead link to the documentation:

The last time it was in the NC 9 documentation (this part still looks to be valid for current setups):
https://docs.nextcloud.com/server/9/admin_manual/installation/installation_wizard.html#setting-strong-directory-permissions