Bootable USB disk ends with a NOGO sign

What do I need to do to use a USB bootable installation drive for my iMac?

The iMac is from 2008 and I tried to reset it to factory settings. The option "re-install OS" does work, but ends with the message that some elements are missing.

Then I made a bootable installation USB drive om an Mac with Catalina installed. The installation file was for High Sierra, the latest version that worked on my iMac. I used all instructions in hehe apple support section.

After starting up and selecting the USB installation disk I get the NOGO sign and the iMac stops.

Does anybody have a suggestion what to do next?