I wonder why I wasn't able to get any results directly on the UA Mileage website when I tried to plug in dates?

And if I wanted to make a stopover in LGA instead of IAD or in addition to, I couldn't because it's not a usual route? I'm allowed one stopover usually, right? I don't remember how but when I flew to GUA, I was allowed to stop in NY.

So to clarify, if I use Star Alliance flights, I have to call to book my miles and thus, pay the fee? But if I can do it on the UA Mileage Plus website, there is no charge?

Thanks for everyone's replies!! You guys are the best!!