How do I confirm if a variable is in a csv file?

I get this error when I run my code and reach the csv file part: line 96, in fieldnames self._fieldnames = next(self.reader) io.UnsupportedOperation: read

My code:

import csv
username = ""
global enteruser
global username

def user():
    age  = input("What is your age:")
    year = input("What is your year group?")
    name = input("What is your name?")
    username = name+age+year
    return username

def username_validator():
    with open('usernames.csv', 'ab') as file:
        reader = csv.DictReader(file)
        for row in reader:
            print(row)
            if  enteruser or username == row["usernames"]: # if the username shall be on column 3 (-> index 2)
                print ("is in file")
                with open("user1.csv","ab") as quiz:
                    quizreader = csv.DictReader(quiz,delimiter=",")
                    for row in quizreader:
                        print(row["name"])
            else:
                print("doesnt work")

isuser = input("Do you have a username?")
if isuser == ("yes" or "Yes"):
    enteruser = input("Enter username:")
    username_validator()
elif isuser == ("no" or "No"):
   user()
else:
    None

print(username)