In hebrew there are two ways to say "enter the": להיכנס אל, and in short: להיכנס ל-.
I think doulingo should except both ways here.
Wait, I thought the verb to enter was להכניס, wouldn't that make the past tense "הכנסה"?
I think it's the past form of the nifal verb נכנס
Ha'chalalit nichnesa letoch ha'chor ha'shachor.
Does Duo ever accept אל? I think I have only seen ל