My mom had lost the ability to walk. Is this a normal thing that happens with Alzheimer’s?

Yes it is common.
There can be a variety of reasons.
"forgetting" how to walk
fear of falling
loss of muscle tone, weakness

My husband went from walking to overnight not walking. It really was not unexpected as I could see him getting weaker as he walked, his gait was becoming more unsteady. Thankfully once he was in the wheelchair he never tried to stand on his own, I used a Sit to Stand then progressed to a Hoyer Lift.

Unfortunately this is normal in advanced dementia. Ask about some PT to see if she can regain some mobility, the inability to ambulate can be a real barrier in care and make remaining at home impossible.