Setup cron job correctly in plesk

Hello i would like to ask something. I am using plesk for my hosting on my vps and installatron for simple one click app installation. Well ive installed nextcloud from installatron and in the settings-main section i can change what type of background job i would like to use (webcron, ajax or cron). I first used ajax for test but i would like to switch to cron-jobs. Now my question: how to corrctly setup cron jobs in plesk for nextcloud? I tried to set it up and it was runned but i have some issues “The config directory is not writable so i set in config file to ‘config_is_read_only’ => true,”. Well what am i doing wrong?