As long as the visits are being tracked, this is not something you should worry about!
Most probably, there is a space or a character missing or added extra in the code, and our code checker cannot find it in the form in which it expects to find it. Or the source-code is not publicly visible (due to an iFrame your website is in, or a redirect from a domain to another) and the checker cannot find it at all.
Rest assured, this can be easily dismissed. Just contact us and we'll have it checked and set on successfully installed, if everything is working properly.