In your code, you first defined N to be a symbolic variable using syms. However, on the very next line you redefined it to be the number 10. This makes y (which you compute using N) a number rather than a symbolic expression. Since the number you pass into solve doesn't contain a symbolic variable, solve doesn't know what you want it to do.
I'm not completely sure what problem you're trying to solve, but leaving N as a symbolic variable (eliminating the line that redefines it to be 10) before calling solve then using subs to substitute a value back for N after solving may do what you want. If it doesn't, please explain in more detail your ultimate goal and we may be able to offer some suggestions for how to achieve that goal.