When attempting to install MacOS on a MacPro (2013) I get an error message

<< the error "installelcapitan.app is not found". >> is because you did not automatically place the "Install MacOS ... .app in the /Applications folder, where it is required to be for the commands to work.

You MUST complete every step of the instructions here, especially step 3 which PLACES 'Install macOS....app' in the expected location:

1. Use these links to download a macOS disk image (.dmg) file. If these links don't work as expected, try again using Safari, which is in your Applications folder.
2. Double-click the .dmg file to open it and see the package (.pkg) file within.
3. Double-click the .pkg file, then follow the onscreen instructions to install the macOS installer into your Applications folder.
4. Open your Applications folder and double-click the macOS installer, named Install [Version Name]. Follow the onscreen installation instructions.

How to download and install macOS - Apple Support

section entitled: "use a web browser... "

Creating the bootable USB stick Installer is a completely separate process from Booting from that created USB stick installer and running the Install. The two are displaced in time, and need not occur on the same machine.

Where are you in that process?

that error suggests you are still trying to make the USB stick.