It's not only acceptable, it's a better translation, because “walk” is an action verb, so present progressive is more likely. For action verbs, English present indicative is only used for habitual action and the narrative present.
The Duolingo staff responsible for translation from Spanish to English don't seem to have fully grasped this language difference yet, and always translate the Spanish present indicative literally as the English present indicative. Unfortunately, this teaches bad English to users who aren't native English speakers.
So please, report every case you encounter where Duolingo inappropriately rejects the English present progressive for a Spanish present indicative action verb. Duolingo staff are gradually correcting these errors when they're reported often enough.
Huh? Nobody's claiming that the present indicative has the same meaning as the present progressive in either language. The point is that, for action verbs, the distinction between the two differs between the two languages.
In English, present progressive is the default form, while present indicative is only used in special circumstances (habitual action and narrative present). In Spanish, the present indicative is the default form, while present progressive is only used in special circumstances (background action and emphatic present).