Example:
Quis custodiet ipsos custodes ?
When i = " " after custodes
a = "uisQay ustodietcay psosiay ustodescay "
b = ""
Then i = ?
So the script go else:
a = a = a + b[1:] + b[0] + "ay" + i
But b is null so index 0 or 1 don't exist.

PS:
In your line if i.isalpha() and i != " " and i != "!":, and i != " " and i != "!" is unuse. If i is a letter, it's not a space or !.
PPS:
a = a + b[1:] + b[0] + "ay", just before return, isn't necessary, when the last character is a ponctuation.

I think the same than you, maybe its correct because they dont put a test in order like you write but its a posibility so for my is not the correct answer

because if you have number<0 you need return 0. Why are you check if i < 0: in your loop?