i am getting Compile error: Argument not optional

You must get that error on this earlier line:

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since Frac is a function that requires an argument and you haven't provided one.

You've given no context whatsoever for what you're doing so it's pretty hard to assist you.

Frac is a function that takes a string and returns a number so I have no idea why you are trying to extract parts of it. You also can't do this:

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because you can't assign a value to a function, only return one from it.

There aren't any fractional values on sheet3. Nor are the relevant values in column C.

Perhaps you want:

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