It sounds like the article "an" is suppressed on the pronunciation...Is it okay?
It’s common for an an between two consonants to be pronounced as an unstressed a.
I am assuming that this would be (idiomatically) something like 'we play on the wayside', rather than 'we play on the journey'?
Though, actually I see that could be wrong, since route could be taken to mean journey as well. :~?
Yes, ar an mbealach can mean either “on the surface of the road” or “en route”.
In that case, why is ‘We play on the road’ marked as wrong?
Because the course creators didn’t anticipate that as a correct answer.
Why's that "m" in "mbealach"
eclipsis due to 'ar', ie, 'ar an bpláta'