To respect the guiline, it should add tests with number and all special characters like :
At six o'clock, the laying of the cable having been successfully accomplished, and the object in view attained; the 'Great Eastern' was once more left in her costly idleness. A French company, called the 'Sea-sickness Company, Limited,' was floated with a capital (in old francs) of 2,000,000.00 F

Use spoiler when you write hints or responses, please.

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.

Ah, I make a mistake. If the last character is a letter, the letter of the whole last word, the word didn't appear. Oops.