Thanks, I didn't notice there was a scrollbar on the right.
I've read somewhere that the beginner practice challenge "Managing expectations" was harder than Skirmish mode, but when I switched to Skirmish mode, I was forced to play with expansion factions I didn't know, so I came back to "Managing expectations" :

And it seems that 96% of players can't pass it, maybe there should be a more progressive tutorial introducing various strategies ?
Also, I'd love to need to adapt my strategy to the map and to what other players do, but it seems like a good build order is enough to win.