Why do I need the a?
sarah- Normally with a verb of action, you use A, tell me someone if I'm wrong.
My dictionary tells me "entrar en algo". Why is "en" not correct here?
I agree, "en" is even more appropriate for this case.
ann- I look in mine and I see that many prepositions can be used with Entrar. Entrar A, entrar en, entrar de.
http://www.fundeu.es/consulta/entrar-a-entrar-en-1921/ According to this page it's en in Spain and a in the Americas. Either is correct.