Pattern error

Before you make the pattern:

View > Guides > Lock Guides

Have you used horizontal or vertical guides when creating the art for your pattern? Or did you either deliberately or accidentally make a selected shape you're using into a guide by going through the View Menu>Guides>Make Guides? In either case you will need to delete any horizontal or vertical and/or release back to live art any guides you may have made from a shape before you can make your art into a pattern. To release a guide back to live art first go under View>Guides>Unlock Guides and then select the shape to release and go View>Guides>Release Guides.

Guides are also objects in AI and have their own layers. That's why for making patterns you need to put guides only on the edges of main object.

For canceling that error do not place guides on main object.

Exactly what are you doing when the message pops up?

You will need to make sure, there's no guide selected when you create a pattern.