Encrypt current Trisquel 9 installation

Are you referring to Full Disk Encryption (FDE) or encryption of particular partition/volume?

Hi!
When installing Trisquel for the first time, the installer procedure (via GUI) ask you if want encrypt all the system, need only follow easy instructions. In this way, need your psw to power on the machine, and need your psw to mount, modify or only watch the partition/s. But is not your case.
For what I know, you can't setup the encryption of a file system using that file system, because need unmount the volume to modify it. You need to run a Live System with the right tools, on a machine do you want to encrypt, and use it to encrypt your Trisquel. If is that your situation, I believe that's the way. Say that because some times I have used UltimateBootCD, an example of Live System build for that stuffs, but I NEVER DO this type of encryption. The are others similar Systems, in text or distro style's, search in distrowatch or distrotest to check.
I don't know other solutions. Any way, do always a great backup, before try any things...

If try UBCD, I can suggest you to use the normal version, from a CD/DVD. Is much complete of tools, start in text mode grab-style, but have also a complete OS in the options. If you can't read CD's, there is an alpha live version for usb stick, but with more less tools.