Answer
Adverbial phrase
Explanation
We wrote in our journals for twenty minutes each day.
The group of words "in our journal" is acting as an adverbial modifying -giving more information about- the verb wrote. Specifically, it is telling us more information about where the action took place; it is answering the question: where did we write? In our journals. Now, it is not a clause because it doesn't have a subject and a verb, so we can conclude that it is an adverbial phrase.