I was getting a random 'u' in my solution and found it converting to utf-8 fixed the problem in case anyone was struggling. First time getting a problem 100% by myself, so I'm pretty proud.
Not the most efficient way and can be shortened.
def findTheDifference(self, s, t):
a = [s[x] for x in range(0,len(s))]
b = [t[x] for x in range(0,len(t))]
for x in a: #a b c d if x in b: #a b c d e b.remove(x) for i in range(0,len(b)): return b[i]