Press F2, which is the algebra menu. Then 'factor'. Press in your expression and you should be set. Give me an example so I may have a better idea of what's going on. Tell me exactly what your are doing, thus resulting in the error message.
I follow the examples in the manul for example on page 50 of the manual which I mimic precisely:
f2 3 ( x - 5 ) ^ 3 ) enter
the response I get back is non-algebraic variable in expression
On page 51 of the manual the factoring example results in the same response as above. Example: f2 2 x ^ 2 - 5 , x )enter.
Thanks for your help