Enabling the debug log on Magento 2.3.1
Since 2.3.1, things have changed a bit. As described in the DevDocs article here, run this command:
php bin/magento setup:config:set --enable-debug-logging=true && php bin/magento cache:flush
Enabling the debug log on Magento 2.2 and above
In Magento 2.2 and above, debugging can be enabled from either the GUI admin panel or from the command line.
First, check which mode your site is in by opening your Magento application directory (the one with index.php) in the shell. Then enter:
php bin/magento deploy:mode:show
If it says "production", only the command line method will work. Otherwise, the GUI method will work too.
GUI method
In the Magento admin panel, go to "Stores" -> "Configuration" -> "Advanced" -> "Developer" -> "Debug" -> "Log to File". Setting this to "Yes" will cause debug information to be logged to var/log/debug.log in your Magento application directory.
After saving the setting, you may get a prompt at the top of the page asking you to flush the cache. If this appears, flush the Magento cache with the link it gives you.
If production mode is enabled (as displayed by php bin/magento deploy:mode:show) then the GUI option will not be available. Note that this the admin panel normally hides this in production mode, so only use this if you're testing.
Command line method
First, open the Magento application directory (the one with index.php) in the shell. To enable debug logging enter:
php bin/magento config:set dev/debug/debug_logging 1 && php bin/magento cache:flush
This will cause debug information to be logged to var/log/debug.log in your Magento application directory. To disable debug logging, enter:
php bin/magento config:set dev/debug/debug_logging 0 && php bin/magento cache:flush