Mac pro is erased, but I can't reinstall MacOS - can I still proceed to trade in?

If your Mac will not start up to a MacOS Welcome or Startup screen, you risk having a not-very-smart technician tell you your Mac is inoperable, and sending it back to you instead of paying.

Invoke Internet Recovery with this key-sequence, and re-install the "shipped in the box" version of MacOS:

• Shift-Option-Command-R: When you press and hold these four keys at startup, macOS Recovery might offer the macOS that came with your Mac, or the closest version still available. 

How to reinstall macOS - Apple Support