How to figure out what error my Java Eclipse project has?

Selecing Window -> Show View -> Problems will open the Problems view, that should have a list of all problems, their location and nature (it can be configured to show only project- or file-local problems, so you may also have to click on the project). Most likely you have a config error in your project, e.g. a reference to a JAR library that cannot be found.

The errors are shown in the Problems view. If it is not opened, go to:

Window > Show View > Problems

Also accessible with the keyboard shortcut Alt+Shift+Q,X

View Errors/Warnings Project based