CASify
How to set up a function
Open the Calculator app on your TI-Nspire CX II CAS and type the name with its variable, so f(x). Then the assignment sign: first a colon, then an equals sign right after it. The expression comes next, and at the end you press enter.
If you'd rather not type the sign, use the menu instead: menu → 1: Actions → 1: Define. The calculator puts the Define command in the entry line, and you keep typing after it. Both ways give you the same result.
Why there's a colon in front
= and := don't mean the same thing, and this is where most people get stuck.
A plain equals sign makes a claim. If you write x=3, you're basically asking the calculator whether that's true. It doesn't know, because x isn't anything specific. The colon in front turns it into an instruction: remember this, from now on f(x) is exactly this expression.
What you can do with it afterwards quickly makes up for the effort. Write f(2) and you get the function value. Write solve(f(x)=0,x) and you get the zeros. You can differentiate, integrate and use it in a graph, and you never have to type the expression again. With a function that takes up half a line, that saves more time and more typos in a test than you'd think.
When the calculator complains
The most common message says that a variable isn't defined. That means you used f before you defined it. Or you've since moved to a different problem or a different document, because definitions only apply where you created them.
The opposite happens just as often. You defined f differently two problems ago, and the old definition still applies. Old definitions don't disappear on their own. Delete the variable before you assign it again, with menu → 1: Actions → 3: Delete Variable or directly with the command DelVar f.
Names have rules. A name can't start with a digit, can't contain spaces and can't be the name of something the calculator already knows. If the calculator tells you the name is reserved, that's exactly what happened.