What is the system log and where is it? at admin/config/workflow/rules/settings one can configure settings for logging related to rules and it states that: "Evaluations errors are logged to the system log".  What and where exactly is the 'system log'?
By default the logs are saved at /admin/reports/dblog for which the dblog module in the core is responsible. More about dblog : https://www.drupal.org/docs/8/core/modules/dblog/overview

You can also have the logs stored on the file system by enabling the syslog module (also in core). More about syslog : https://www.drupal.org/docs/8/core/modules/syslog/overview

It is recommended that you should not use the dblog module if you need to log messages more frequently as it does a write operation to the database.