Failed to start mysql57.service: Unit not found.

This error message suggests that the MySQL service unit cannot be found on your system. There could be a few reasons for this. Here are some steps you can try to resolve the issue:

1. Verify MySQL installation: Ensure that MySQL is properly installed on your system. You can do this by running the following command:

   mysql --version

2. Check service name: Confirm the correct name of the MySQL service unit. Depending on your system and setup, the service name might differ. Common service names include `mysql`, `mysqld`, or `mariadb`. You can use the following command to list all available service units:

   systemctl list-unit-files | grep mysql

3. Restart the service: Try restarting the MySQL service using the appropriate service name. For example, if the service name is `mysql`, you can run the following command:

   sudo systemctl restart mysql

4. Reinstall MySQL: If the previous steps don't resolve the issue, you may need to reinstall MySQL. Make sure to backup any important data before proceeding.

If none of these steps work, it would be helpful to provide more information about your system and any recent changes you made that could have impacted the MySQL service.