Eclipse shows errors but I can't find them

Take a look at

Window → Show View → Problems

or

Window → Show View → Error Log

Based on the error you showed ('footballforum' is missing required Java project: 'ApiDemos'), I would check your build path. Right-click the footballforum project and choose Build Path > Configure Build Path. Make sure ApiDemos is on the projects tab of the build path options.

This happens from time to time in Eclipse. In the "Project" menu there's a "Clean" option, that usually takes care of the problem.