If there is a difference, it's subtle. Trajet are Voyage are considered synonyms by this site:
Sure, and DL should accept commences, but it's worth noting that there is quite a difference in register - "commence" in French is an extremely common, ordinary word - like "start" - while "commence" in English is more elevated in tone: a good deal more formal, literary, old-fashioned. These things matter when you are actually translating, although Duo doesn't usually take much note of such things in these lessons.
The conjugation in two of my French verb books, has the present for Tu, and I presume ton, spelled commences, with an s on the end. But their answer dropped the s, and marked me wrong. So, DL, which is it? I find the grammar on this French site somewhat shaky, although it is good in most other ways. Are the DL instructors from places other than France? Seriously.