Nextcloud Log File
I was able to locate documentation that may be able to assist you with locating the log file. It looks like there's a setting in the "config/config.php" file, based on the documentation. I would also check the "messages" and/or "syslog" files for possible insight.
If this doesn't help, you can try using the "find" command to manually locate the log file. An example I would try is the following:
sudo find / -name nextcloud*