"Build path entry is missing" error when trying to create a new project in Eclipse

This error is due to JRE System Library. Maybe you didn't give appropriate JRE for eclipse project to run. SO follow below steps.

1. Right-click on the project and choose properties.
2. Click on the Java Build Path option in the left side menu.
3. From the Java Build Path window, click on the Libraries Tab.
4. Make sure JRE System library is listed, if it is not listed then you can add, by clicking "Add Library" from the right side menu.
5. So if JRE System Library is already listed then double click on JRE System Library which was showing error on Java Build Path window Libraries tab previously .
6. Then it will open another window called JRE System Library . So in that window choose Alternate JRE . From that drop down choose your JRE.

In my case, it is java-8-openjdk-amd64 , as I am using ubuntu 16.04. Like that you can also choose your JRE System library .

It is an old question, but I solved it by modifying JRE system library and select a valid Execution environment in my case (Mac OSX) JRE 1.6