Does an American have an obligation to vote?

Certainly not. That's part of what comes from democracy: The right to choose something, as opposed to being forced into a choice. Ask millions of citizens living in totalitarian countries around the world!
And a lot of people would say, "Don't vote at all", which is about as far from a democratic ideal as one could get! There's an old saying by Sir Winston Churchill:

"Democracy is the worst form of government ever invented, but it's the best the world has seen yet".

In other words, until something better comes along, why don't we throw the brakes on the bulldozers for a minute?
Political and social change is a must. There are a lot of injustices, imbalances, and corrections to the system need to be made. But tear it all down, what do we have? We regress into past years, regardless of technology! If the US government were to somehow fail, it would be catastrophic for the world!!

For someone who's 20 years old, I get it.

At that age, you're not always thinking about that stuff. But for guys 30 something and 40 and up, what's your excuse? Should have enough sense now to know at least some of what I'm saying! Either that or you really just don't care period!
Whether Libertarian, Democratic or even Republican (or Trump Party), go out and vote on November 6th. It's the one voice we have in this democracy!