It means that there are errors and you'll have to fix them.

In this case, if it says the "Addin" has child elements with issues, look at the right pane. There should be a tab there that says "Addin" on it. Click it, and you'll see a bunch of red X's where the problems are. Basically, you'll have to add in info for the .goomod so people can play it.

Video tutorial for that here:

In case you can't view the video, just go to the "addin" tab and look for elements with the red X's. Click on them, and there should be directions in the lower left corner of the screen for what you should put there.