"Visit Canada in the summer!"
Translation:Vizitu Kanadon en somero!
That could work, and it would work in conversation. Duo is trying to show us that there are places where leaving the la out can still be grammatical and comprehendible.
As Meera_Stm pointed out this could have also been somere which would make the trip to Canada a regular thing. But the sentence, as written, is more of an advertising slogan, "Visit Canada in Summer."
I'm certain the pictures associated with that slogan show Banff, Vancouver's Rose Garden, and the Château Frontenac, but nothing which shows you the mosquitos.
Mi ja volis viziti Kanadon ĉi-somere, sed pro la koronvirusa krizo UEA planas prokraston de la Universala Kongreso en Montrealo, do mi verŝajne ne baldaŭ iros tie.
Do mi restu hejme kaj lernu Esperanton.
I blame it on usage. But often when a country (or state) has an ethnicity, region or city within it with the same (or similar) name, that ethnicity, region or city gets the simple ~o ending and the country gets the ~io. European countries all seem to get the ~io, and elsewhere in the world it seems to be a coin toss.
When Esperanto started all countries, with a few notable exceptions, were ~ujo, or ~lando. But people didn't like that their country was a "container for" X people (Francio used to be Francujo, Germanio used to be Germanujo, ktp, as if one could keep the French and Germans contained?) So it was determined that ~io would replace the ~ujo on countries, and I think Esperanto has been mildly confused by this change ever since.
Marko Kramer's answer to the Esperanto Stack Exchange question Why are country names in Esperanto so “irregular”? explains it quite well.
Country names in Esperanto aren't more irregular than Esperanto names for other classes of things, e.g., tools. (Some of which end in -ilo, some of which don't.) It's just easier to notice that "irregularity" about countries, because you usually learn lots of country names in a single lesson, but the words for various tools are spread out over different lessons.