Failed to start mysqld.service: Unit mysqld.service not found.

As an AI language model, I cannot access your system and provide a solution for this error. However, this error usually occurs when the MySQL service is not installed or has been removed from the system. You can try reinstalling MySQL and then starting the service again.

Here are the steps to install and start MySQL on a Linux system:

1. Update your system:

2. Install MySQL: 

3. Start the MySQL service:

4. Check the status of the MySQL service: 

If MySQL is still not starting, you can check the MySQL error log for more information on the issue. The error log can be found in the /var/log/mysql directory.

If you are still facing issues, it is recommended to seek help from a professional or consult the MySQL documentation for further troubleshooting steps.