Unit nginx.service could not be found.

This error message usually occurs when you try to start or stop an NGINX service but the service is not installed on your system or the service name is incorrect.

To fix this issue, you can try the following steps:

1. Check if NGINX is installed on your system by running the command: `sudo systemctl status nginx`

2. If NGINX is not installed, you can install it using the command: `sudo apt-get install nginx`

3. Once installed, you can start the NGINX service using the command: `sudo systemctl start nginx`

4. To stop the NGINX service, use the command: `sudo systemctl stop nginx`

5. If you are still having issues, verify the correct service name for NGINX on your system by running the command: `systemctl list-units --type=service`

If none of the above steps work, you may need to check your system logs for more details on why the service is not found.