Hi Ricky, welcome. OK, first thing is, API means Application Programming Interface, which is how Auctiva communicates with eBay. So, if you see an API error message, it means that whatever you tried to do failed to 'pass muster' through that interface.
Now, if you go to your Auctiva 'My Account' tab up at the right there, and look to see if you have any events (left hand side). Look for any failures, and hopefully you will see an explanation of the failure. Actually, immediately above the My Accounts tab, on every page, is a little events counter, with 3 numbers, a green one, an amber one, and a red one - anything in red means there was a problem, clicking the red number will take you to the events page too.
Failing that, open up your listing in Auctiva, and click the 'Error Check' link, a blue button right down the bottom - after it has done it's thing, you should either get a green all ok, or a red, this is wrong, at the top of the page.
I suspect that this is a simple error in your listing, and easily fixed once you know what you are looking for. Let us know how you get on.