Ah, so you don't "constantly" get the error. ;-)
What you've described is a timing issue. I'm guessing, from your description, that there's some action or event performed before your attempt to click the "Add Link" button which should enable that button (like filling in a text field, highlighting something, etc).
So, my guess is that you need to add some sort of wait method or process in before your attempt to click the "Add Link" to wait until it is enabled before you attempt to click on it. Look into using WaitProperty on the button to wait for Enabled to be true.