I use Rufus all the time to make bootable disks with no issues. If you are intending to replace Win11 with Truenas then use Rufus, but allow it to build the bootable usb using the defaults to make disk. No special settings are needed.
On Secure boot machines the Secure boot needs to be turned off in the BIOS, in order to boot from a usb stick; but I see that was already done.
Truenas is an storage appliance software that is based on Debian (TNScale) and installs on bare metal. It is not a Linux distribution like Ubuntu, Fedora etc. is and cannot be duel booted like regular Linux distributions can. The install will use the entire disk selected for the boot disk and that disk is not ever available for data storage. The selected disk will be formatted and repartitioned for Truenas requirements. For any install to be useable you will need a minimum of 2 disks. One for Boot and one for Data.
If you just want to give Truenas a try to see what Truenas is all about, learn, and test, you can install Virtualbox in windows 11 and create a virtual machine using the Truenas iso, no Rufus required just select the iso during the VM initial setup. The virtual machine can pretty much do everything a regular bare metal install can do. Just create several thin provisioned virtual disks and attach them from the VM setup. Use one for boot, and 3, 4 , or 5 additional virtual disks for data disks. 20GB thin provisioned sizes are fine for all and take very little physical drive space to begin with.
I use a VirtualBox Truenas Scale VM to learn and test how to do various things to make sure I understand everything before applying it to my production server. That way if I mess something up I can just revert to a previous snapshot of the VM with no harm done.
