UnboundLocalError: local variable 'mp' referenced before assignment

I get this error when testing:

Please enter the customer's name: 1
Please enter the customer's Phone Number:01506890639
Please enter the size of group: 4
Please enter the rating of the meal: 3
Do you wish to continue: Y/N?n


The largest group that visited had a cohort of:4
Traceback (most recent call last):
  File "E:/CWtsk/code2.1.py", line 93, in <module>
    main()
  File "E:/CWtsk/code2.1.py", line 12, in main
    mealrating(score)
  File "E:/CWtsk/code2.1.py", line 67, in mealrating
    mp = mp + 1
UnboundLocalError: local variable 'mp' referenced before assignment

I’m unsure about the reference thing because as you can see in my code, it is not referenced before.

def main():
    name = []
    phone = []
    groupno = []
    score = []
    review = []
    q="y"


    data(name, phone, groupno, score, q)
    maxi(groupno)
    mealrating(score)
    poorest(name, phone, groupno, score, review)

# Gathering Data ---------------------------------------------------

def data(n, p, g, s, q):

    while q != "n":
        name =  input("Please enter the customer's name: ")
        n.append(name)


        phone = ""
        while len(str(phone)) != 11:
            try:
                phone = input("Please enter the customer's Phone Number:")
            except ValueError:
                phone = ""
        p.append(phone)


        groupno = int(input('Please enter the size of group: '))
        while groupno < 1 or groupno > 20:
            groupsizes= int(input('Please enter a valid group size: '))
        g.append(groupno)


        score = int(input('Please enter the rating of the meal: '))
        while score < 1 or score > 10:
            score = int(input('Please enter the rating of the meal- between 1 and 10: '))
        s.append(score)

        q=input("Do you wish to continue: Y/N?")

# finding largest group size ---------------------------------------

def maxi(groupno):

    xGroup = groupno[0]
    for x in range(0,len(groupno)):
        if groupno[x] > xGroup:
            xGroup = groupno[x]
    print('n')
    print('The largest group that visited had a cohort of:' + str(xGroup))

# determining the meal rating and counting number of applied ratings and priniting --------------------------------------------------------------------------------

def mealrating(score):
    for x in range(0,len(score)):
        if score[x] >= 1 and score[x] <= 3:
            mp = mp + 1

            if score[x] >= 4 and score[x] <= 6:
                mg = mg + 1

                if score[x] >= 7 and score[x] <= 10:
                    me = me + 1

        print('n')
        print("The customer rated tonight's meal as:")
        print('%10s' % ('Poor:', mp ))
        print('%10s' % ('Good:', mg ))
        print('%10s' % ('Excellent:', me ))

# finding the poor meals -------------------------------------------

def poorest(name, score, review):
    for x in range (len.name):
        if review[x] == poor:
            print("Contact Name: " % name[x])
            print("Phone Number: " % phone[x])
            print("Rating: " % score[x])


main()

I’m not sure what this is. Although I need to adjust the last the last function as I forgot about it trying to troubleshoot.