If you want to create a bootable pen drive for Google chrome os Flex but you don't have access to windows system then you can follow the method below :
1. Download the linux_recovery.sh from https://dl.google.com/dl/edgedl/chromeos/recovery/linux_recovery.sh
2. Open a terminal and run below command
sudo bash linux_recovery.sh --config https://dl.google.com/dl/edgedl/chromeos/recovery/cloudready_recovery.conf
3. Follow the instructions to select the pen drive. It will take some time to download and copy the image.
once done you can restart your computer and choose the pen drive to boot chrome os flex.
References:
https://support.google.com/chromebook/answer/1080595?hl=en#zippy=%2Cuse-a-linux-computer
https://support.google.com/chromeosflex/thread/191157505?hl=en&msgid=191238953
