The MySQL driver that's included in Drupal 9 core requires MySQL 5.7 or higher.

This database driver lets you install and use beta, RC, and stable releases of Drupal 9.0 on MySQL 5.6. This driver will only be maintained until MySQL 5.6's EOL date of Feb. 2021, and only for Drupal 9.0. It will not work on Drupal 9.1.