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The log files can be found in the logs/ directory in frappe-bench (the directory where all server bench commands are run), so for example
frappe@ubuntu1804lts:~/frappe-bench$ pwd
/home/frappe/frappe-bench
frappe@ubuntu1804lts:~/frappe-bench$ bench version
erpnext 12.6.0
frappe 12.4.1
frappe@ubuntu1804lts:~/frappe-bench$ find logs -name ‘*.log’ | xargs ls -al
-rw-r–r-- 1 frappe frappe 18720 Apr 1 06:42 logs/bench.log
-rw-r–r-- 1 frappe frappe 77824 May 5 00:33 logs/frappe.log
-rw-r–r-- 1 root root 21143552 Mar 19 16:24 logs/node-socketio.error.log
-rw-r–r-- 1 root root 162368 May 4 19:47 logs/node-socketio.log
-rw-r–r-- 1 root root 0 Jan 12 01:08 logs/redis-cache.error.log
-rw-r–r-- 1 root root 62636 May 4 19:46 logs/redis-cache.log
-rw-r–r-- 1 root root 0 Jan 12 01:08 logs/redis-queue.error.log
-rw-r–r-- 1 root root 64560 May 4 19:46 logs/redis-queue.log
-rw-r–r-- 1 root root 0 Jan 12 01:08 logs/redis-socketio.error.log
-rw-r–r-- 1 root root 62636 May 4 19:46 logs/redis-socketio.log
-rw-r–r-- 1 root root 0 Jan 12 01:08 logs/schedule.error.log
-rw-r–r-- 1 root root 9803 Feb 3 20:45 logs/schedule.log
-rw-r–r-- 1 root root 37860139 May 4 19:47 logs/web.error.log
-rw-r–r-- 1 root root 63973 Apr 22 15:46 logs/web.log
-rw-r–r-- 1 root root 44925493 May 5 00:32 logs/worker.error.log
-rw-r–r-- 1 root root 0 Jan 12 01:08 logs/worker.log
edit: Other logs to check include those that relate to the OS here:
root@ubuntu:~# ls -alt /var/log
total 3784
-rw-r----- 1 syslog adm 161644 May 5 10:14 kern.log
-rw-r----- 1 syslog adm 22683 May 5 10:14 syslog
-rw-r----- 1 syslog adm 161644 May 5 10:14 ufw.log
-rw-r----- 1 syslog adm 8321 May 5 09:17 auth.log
-rw-r----- 1 syslog adm 3491 May 5 09:17 cron.log
drwx------ 2 root root 4096 May 5 08:05 letsencrypt