installing macos from a installer usb stick

rolffromza wrote:

installer app opens, but when entering "agreed" and "agreed" again, error message "An error occurred while preparing the installation".

installer worked perfectly on previous occasions

This is predicated on the user having successfully created USB Bootable Drive of macOS Sequoia

To perform this action will require booting from a Bootable Installer. 

The Bootable Installer can Only be performed on an Apple Computer 

This will have to be performed from a Qualifying Computer to run the version of macOS to be made on the Bootable Installer.

Example : Bootable Installer of Sequoia would have to be done on a computer that Qualifies to run Sequoia.

If an error occurred while updating or installing macOS

Check storage space

If the message says that your Mac doesn't have enough storage space, free up storage space.

Check your internet connection

The installer needs a stable internet connection to download macOS and get firmware and other information specific to your Mac model. Make sure that your Mac is connected to the internet.

Also, keep in mind that using a USB stick is not a reliable medium. They are subject to failure. That might be your issue.