How to reinstate Mac HD so I can reinstall OS on older iMac.

I have an old iMac at work (not sure of date) It was working well until auto update tried to intstall Catalina. It failed and wouldn’t boot. I went into disk utility and tried to restore it but have ended up losing the Mac HD. I’ve made a USB installer. I know very little about the technical aspects but would like to try to fix this. It isn’t supported by IT where I work, as it was a donation. Do I need to type code into Terminal, any suggestions welcome. I realise I could take it to Apple but since I don’t own it, I don’t want to spend on it.