"It is open until 8 pm."
おみせはあきます ー The shop will open. / The shop opens. (i.e. the shop is capable of opening)
おみせはあいています ー The shop is open. (i.e. the shop is in the ongoing state of being open)
Never mind, I found out it didn't get accepted just because I didn't put 午後
In English "open" functions as an adjective in this context, while Japanese uses the verb 開く in its て-form with いる to show that it has opened and is still in that state.