@mooncaptain and @dma_pf
Thank you !!
After reading your previous reply, ( and after having removed the WireGuard package, with the option not to save the previous configurations ). I reinstalled the package, and the first thing I did was, as mentioned by @mooncaptain, I went to the 'Settings' tab on the package, and enable it. I tried to do that on the previous attempt but was unable to do that.
After doing that, I created the tunnel and checked to see if it appears on the selection on the interface, and this time it did !.
I must have done something right when removing the package, without any previous settings.
BTW, I didn't reboot the FW, as I have an offsite backup running that cannot be interrupted at this time.
So this time I can select and assign the interface!
Thank for the video, I'm going to watch it before I continue with this setup.
I'll update this after that finishes.
Thanks again.
JG