How to boot Debian installer from PureBoot?

Edit: when doing a factory reset I get an error saying:

Unable to mount USB on /media ( no idea what I did to change a location)

Mount: mounting /dev/sda on /media failed: no medium found.

On the second advices instructions:

When I go to options -> gpg options -> generate gpg keys manually on librem key.

It asks for me to type admin and then generate.

I ask it to replace the existing keys by selecting: y.

It then errors and says:

Gpg error clearing forced signature pin flag: no pinentry.

At no point did I see when I should have entered my pin (which is currently default)