Need to build a bootable Catalina thumb drive for my "Late 2015" iMac

Want ( to try ) to sell my old iMac. Followed the Apple instructions on how to prep it for sale and wiped the disk. Must have done something wrong because i can't restore the operating system ( it came with Sierra ) from the recovery partition or via internet connection. I have a "late 2019" iMac ( intel I9 ) running Sonoma and an M3 MacBook Air running Sequoia. Read the Apple instructions on how to build a bootable Catalina thumbdrive but can't get it to work. Downloaded the Catalina Installer (installOS.dmg) but it won't run on either of my macs. Error message i am getting is that the Catalina version of installOS is not supported on either of my macs.

What am i doing wrong ?