This is the 13611th most frequent Russian word.
недостающую
"Недостающую" means "missing" or "lacking" (feminine accusative form).
The word 'недостающую' is used as an adjective in the accusative case, modifying the noun 'страницу'.
Учитель попросил найти недостающую страницу в книге.
The teacher asked to find the missing page in the book.
Here, 'недостающую' functions as an adjective describing the noun 'информацию' in the accusative case.
Мы заполнили недостающую информацию в форме.
We filled in the missing information in the form.
In this sentence, 'недостающую' is an adjective modifying 'деталь', and indicates something necessary but absent being added.