The logging needs to be enabled in Magento 2 in several places. Follow these steps:

  1. Log in to the Magento Admin Panel.
  2. Enable the logging in the Magento extension (option "Logging" , "Debug" or similar)
  3. Then log in to the server via SSH.
  4. Navigate into the Magento base directory.
  5. Execute the following commands as the Magento filesystem owner (e.g. www-data):
bin/magento config:set dev/debug/debug_logging 1
bin/magento cache:flush

### NOTE: For Magento 2.2.8+ or 2.3.1+ do this instead:
bin/magento setup:config:set --enable-debug-logging=true
bin/magento cache:flush

The log messages are written to"var/log/debug.log". If in doubt, look at the timestamps of the log files.


To disable the logging:

bin/magento config:set dev/debug/debug_logging 0
bin/magento cache:flush

### NOTE: For Magento 2.2.8+ or 2.3.1+ do this instead:
bin/magento setup:config:set --enable-debug-logging=false
bin/magento cache:flush

See also Magento 2 DevDocs: Logging.