"If it rains tomorrow, then we won't go out" should also be accepted, as "If... then..." is a common sentence construction in English

"If it rains tomorrow, then we won't go out" 如果。。就

If it is raining tomorrow = If it rains tomorrow?

This should also accept "If it rains tomorrow we just won't go out". That's natural in English and covers the 就.

"outside" should also be acceptable in place of "out"

The character for 'then' appears in the Chinese text. It should be accepted in the English response.

"If it rains tomorrow, then we will not go out" should also be accepted as answer

You should add "then" to the correct answer