@Chompo Might be curious as to the error..
I looked on a 4.8 machine that is close - apt search sagemath showed it in the repo..
I ran the install command apt install sagemath and a list of dependancies were displayed along with the ... do you want to install... which I cancelled
I did test on my LL6.2 VM and the install was the same. I would want to assume the same for LL5.x
Again maybe the error will help as to the why.
Maybe try running a Lite Update prior may help as well.