This compilation unit is not on the build path of a Java project

Since you imported the project as a General Project, it does not have the java nature and that is the problem.

Add the below lines in the .project file of your workspace and refresh.

<natures>
      <nature>org.eclipse.jdt.core.javanature</nature>
</natures>

Another alternative to Loganathan Mohanraj's solution (which effectively does the same, but from the GUI):

1. Right-Click on your project
2. Go to "Properties"
3. Choose "Project Natures"
4. Click on "Add"
5. Choose "Java"
6. Click "Apply and Close"

If it is a Maven project then sometimes re-importing of it helps:

1. Right-click the project in the Project Explorer and choose Delete.
2. File > Import... > Maven > Existing Maven Projects > Next > Root Directory > Browse your project from Disk.

Hope it will resolve the issue.