unit mysqld.service could not be found

This error message typically indicates that the mysqld service is not installed or is not currently running on your system. To resolve this issue, you may need to install the MySQL server and start the mysqld service. Here are some steps you can follow:

1. Install the MySQL server:
- On Ubuntu or Debian, you can use the following command: `sudo apt-get install mysql-server`
- On CentOS, Fedora, or Red Hat, you can use the following command: `sudo yum install mysql-server`
2. Start the mysqld service:
- On Ubuntu or Debian, you can use the following command: `sudo systemctl start mysql`
- On CentOS, Fedora, or Red Hat, you can use the following command: `sudo systemctl start mysqld`

Once the service is started, you can check its status using the following command: `sudo systemctl status mysql` or `sudo systemctl status mysqld`. If the service is running properly, you should see a message indicating that the service is active and running.