Can letrozole, or any AI, cause degenerative disc disease? I have not seen research that determines this, only one case study.
I was looking at lumbar area pain that might be caused by something other than cancer. It is well known that degenerative disc disease is much higher in postmenopausal women. Due to the loss of estrogen, see below. If someone already had problems with degenerative discs, symptomatic or not, it stands to reason that an AI may exacerbate it.
Does an AI cause degenerative disc disease? Probably not. Can it worsen it? Probably so.

Interestingly, looking for answers to the above question I found an article stating that in their study - women on diuretics had less joint pain while taking an AI. It was suggested that the joint pain was caused by increased fluid in the joint. I had not read this before, maybe those taking AIs already know this. But if not, might be something to discuss with your doctor.

Sex hormones and degenerative disc disease
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10556885/
The role of diuretics with AIs
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6428535/