There could any number of reasons as to why you may see a Compile Error (yellow box below) when trying to compile your Blueprint.
In the event that this does occur, if you try to play your game, you will see a message similar to below.
The Blueprint that has the error will also be reflected in the Level Editor viewport as well.
In this instance, you may not be able to successfully use the debugging methods to determine what is occurring. Inside the Blueprint that you compiled that is giving you the error, the nodes that are causing the error will be denoted for you to help identify where the issue is occurring.
You can however refer to the Compiler Results window for additional information as to why you are having an error.
The Compiler Results window should pop up automatically when you encounter an error. You can click on the magnifying class to jump to the location of the error and the node in question. To help solve the issue, a text description of the error is also displayed.