High Sierra upgrade to Mojave or Catalina - Recovery server not found
I am trying to upgrade my MacOS (2012 - 10.13.6) to either Mojave or Catalina. I keep getting a message that the recovery server cannot be contacted (for both OS systems). What's the work-around? Everything is backed up to Time Machine - just need to upgrade so i can add Final Cut Pro. Thank you.
iMac 27″, macOS 10.13