I had this problem a few years ago.
I took the hard drive out of the laptop I installed it on and put it in a
laptop that reports PAE as a CPU feature. The first laptop actually supports
PAE, but it doesn't correctly announce it as a feature. If your laptop really
doesn't support PAE, then this may not work.
I then installed Trisquel 6 (the latest available then) and then installed a
package called fake-pae. I swapped out the drive and it booted fine. I never
tried booting it without the fake-pae package, so I don't know if it would
have worked without it or not.
Here's the thread I made for reference:
https://trisquel.info/en/forum/boot-trisquel-computers-without-pae
