In this sentence:
La maleta era muy pequeña y no pude poner todo en ella.
The suitcase was very small and I couldn't put everything in it.
Todo here is detached from the noun maleta; it's not modifying that noun. Here todo refers to "everything". There is a bullet point in the lesson by the end that talks about "todo" referring to "everything" (referring to an idea or a group of things seen as a unit).