This question is asked multiple times a day, and like everyone else that asks this question you haven't told us the most important part of information....what is in iString. Because what is in iString isn't in the format that your ParseExact is asking for. That's exactly what the error message is telling you.
The only thing I can think of is that you don't know what the date format strings are. "tt", for example, is "AM" or "PM" so you are expecting a date like
22/01/2016 01:26:PM
It's unlikely your date is actually in this format, maybe you meant "ss" instead of "tt".
Custom Date and Time Format Strings[
^]
Unless you post what is in iString there really is nothing anyone can tell you.