Voir is generally translated as to see and regarder as to watch. In English we could say that we saw a video or that we watched a video, and I believe in French, you could say either j'ai vu or j'ai regardé une video. You can always report it as a translation. I believe this one of the new sentences added to the French tree so they might not have all the acceptable translations added to the system yet. Bonne continuation!
why is watched not acceptable? It's what we say in English. I've also long since given up the battle about awesome. Those who object and call it teen age slang are quite correct, but Duolingo will not budge on this point. This program has so little respect for what is called Standard English while being incredibly picky about French usage that it often makes me very angry.