"He found sand in the shoe."
Translation:O, ayakkabının içinde kum buldu.
It is ayakkabıda. You're right that it should have an "n", but it doesn't. Some words are merged to such an extent that they're not perceived as compound nouns anymore, in standard Turkish.
In my dialect, we say "pazartesine kadar" for example, but the correct form nowadays is "pazartesiYe kadar". My grandma could say "kahvaltıNdan sonra dışarı çıktım" with an N, which is also not done in today's Turkish.