Trying to erase iMac to give to daughter...

Installed Catalina a few weeks ago on iMac (2015, I think). Husband bought me a new iMac for Christmas, so went through the process of erasing the hard disk to give the old girl to my daughter. In recovery mode, as directions indicated, but "Reinstall macOS" returns an error message... The target volume [Macintosh HD] is part of incomplete system and can not be installed to." Will not reinstall macOS Catalina, tried restarting? Also, tried internal TM recovery, but that just starting and restoring my computer with a error message. Please help, these computers are to helpful to become a junk pile thanks to Catalina???