Eclipse is erroring because if you try and create a project on a directory that exists, Eclipse doesn’t know if it’s an actual project or not – so it errors, saving you from losing work!
So you have two solutions:
Move the folder
counter_srcsomewhere else, then create the project (which will create the directory), then import the source files back into the newly created
Right-click on the project explorer and import an existing project, select
C:\Users\Martin\Java\Counter\as your root directory. If Eclipse sees a project, you will be able to import it.