"An zwei Morgen der Woche gehe ich eine Stunde spazieren."
"Ich gehe spazieren" – it's nominative singular. Wir gehen spazieren would be plural.
Duo people don't seem to have a lot of foresight. They should either not include sentences that have literally 20 different translations, or spend some time coding in each possible one.
"I go on a walk" was marked wrong whereas "I go for a walk" was in the translation. >.>
Also, no one says "two mornings a week" in english.
? Lots of people say "two mornings a week," provided of course that they have something to do two mornings a week. It's very similar to saying "twice a week" or "three times a week."
"Two mornings a week" is commonly used in my experience. It clarifies the actual time as morning instead of any time of day.