install – Big Sur bootable installer cannot be created or corrupted after creation

I have an iMac 27″ 2015 that I want to do a clean install of Big Sur on it.

I followed the tutorial on creating the bootable installation media but it failed with an error: “/Applications/Install macOS Big does not appear to be a valid OS installer application.”

I found a question like this here that said that you must start the installation and let it download the missing files and when it asks to restart you just quit the installer and create the bootable installer again.

I followed the instructions and was able to create the installation media and I restarted the iMac and booted into the installation drive and erased the fusion drive and started to install the macOS.

After choosing the fusion drive to install the OS and accepting the EULA, I had an error message that says that it cannot continue installation because the package is corrupted

I tried it two times and both times the same corrupted error and I wanted to ask is there a sure and tried way to create an install media for Big Sur?