Taking a look at your expression from your notebook, it is of the general form:

a q^(? - 1) + b q^? == 1

This generally will not have a closed form solution for arbitrary

**?**. The error message (actually an informative warning message generateted by

**Solve**) is simply reflecting this fact.

(By the way, I am guessing that your e in your expression is meant to be the number

**E** in Mathematica. But that is irrelevant to your question as those factors are overall factors in your expression and don't affect the discussion.)