Odd issue, haven’t experienced it myself. Can I ask which version of imager you have (should show on Pi imager)
I can see if I can recreate issue to help fix. Another way is to flash via https://etcher.balena.io/ but if I know which Pi imager you’re using as I haven’t seen that issue before
i.e the .img file inside the extracted folder ( the folder that was extracted from the .gz file )
It may have been that I chose the incorrect option in 7zip to extract the folder which was causing the issue I’m not sure.
The menu options of 7zip were in Spanish so I maybe didn’t choose the right one ( I think I chose “Extract here” and the other option was “Extract Archive”)
It could be nice to put those ( *** simple but important *** ) steps on the PiFi guide doc ( bcos I see related posts on related forums about similiar error messages )