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Yes I am confused as well.

If you receive the wrong thing then you open a claim for not as described as it doesn't match the ad.

But do you mean you ordered and have now found the order is for a 6 string guitar and you want to cancel?

Have another look at the ad, I am like sandy and wondering if it was one of those multiple listings with a  drop down menu and you inadvertently clicked on the 6 string option. Check the ad carefully, look at the prices and see if there is a 6 string option and price plus a 12 string option and price (which i presume would be dearer). It is easy to make a mistake sometimes, I have done it myself.

If this happened, I'd be inclined to message the seller and say I made a mistake and actually wanted the 12 string version and they may be able to fix things for you by having you pay the difference or by cancelling to let you rebuy, I don't know.

*Too late for me to edit 

Wait.......   you ordered it as in you have already received it and it was wrong,

or ordered from a multi-listing and there was no '12 string' option

or the heading said 12 string and the description said 6

Something else?

I am confused if a 12 string was advertised and when you went to buy it, it then said 6 string why you would go ahead with the purchase?

What is the item number 

Why would you be trying to cancel when what you should be doing is opening an item not as described dispute (and follow through with it?

You will not get you money back if you fail to follow the correct process 

You do not 'have the option' because it is not they way to resolve it