What does "The creative type contradicts to the optimization goal" mean?

Full error text: 

The creative type contradicts to the optimization goal. The type of your creative has to be consistent with your optimization goal. Also please make sure your optimization goal is not NONE.

This error occurs when the promoted object is not corresponding with your advertising objective. For example, you would like to promote a Mobile App and instead of choosing the Mobile App Installs objective you chose Website Clicks and insert a link, redirecting users to App Store or Google Store. 

To solve this problem you just need to choose the correct advertising objective (in the given example - Mobile App Installs and insert a direct link to App Store or Google Store).