It's because your not transfer your array to list. For have chance reuse array with map function we must transfer it to list:

if name == 'main': n = int(input()) arr = list(map(int, input().split())) first = 0 second = 0

for i in arr:
    if i > first:
        first = i

for j in arr:
    if j == first:
        continue
    elif j > second:
        second = j

print(second)