Here is my implementation for the pagerank pset for cs50 ai

pagerank.py

def sample_pagerank(corpus, damping_factor, n):
    for i in corpus:
        pageRank[i] = 0

    weights = [0]*len(corpus)
    count = n
    while count != 0:
        count -= 1
        sample = random.choices(list(corpus),weights)[0]
        weights = list(transition_model(corpus,sample,damping_factor).values())
        pageRank[sample] = pageRank[sample] + 1

    for i in pageRank:
        pageRank[i] /= n

    return pageRank

Before manipulating pageRank on line 3, you should define it, is it a list (array) or a dict?
Try pageRank = {} to create an empty dictionary before the loop on line 2.