There is an error with fptr.write(result + '\n'). Change it to fptr.write(str(result) + '\n'). The correct code is below.

    def solve(n):
        if 9 % n == 0:
                print(9)
                return 9

        curr_str = ['9']
        next_str = []
        found = False

        while not found:
            for curr in curr_str:
                    a = curr + '0'
                    b = curr + '9'

                    if int(a) % n == 0:
                            print(int(a))
                            found = True
                            return int(a)

                    if int(b) % n == 0:
                            print(int(b))
                            found = True
                            return int(b)

                    next_str.append(str(a))
                    next_str.append(str(b))

        curr_str = next_str
        next_str = []

if name == 'main': fptr = open(os.environ['OUTPUT_PATH'], 'w')

t = int(input().strip())

for t_itr in range(t):
    n = int(input().strip())

    result = solve(n)
    # fptr.write(result + '\n')
    # add str(result) because otherwise it does not work
    fptr.write(str(result) + '\n')

fptr.close()