"Tentara itu tidak bisa membeli sepatu" is WRONG.
"Prajurit itu tidak bisa membeli sepatu-sepatu" is also WRONG.
Both sentences are actually CORRECT.
I am an Indonesian native speaker.
Only s****d people say that both are WRONG. Sorry to say that.
"Serdadu itu tidak bisa membeli sepatu" is also WRONG.
This is also CORRECT.
The alternate answer (prajurit) doesn't make your answer correct. This BUG is intolerable.