When I clicked "Estimate your organization's code coverage" in Apex Classes from Setup Menu,
I got the following error.
Why this error coming? Is there any other alternate approach to find my organization's overall code coverage? Please suggest.
Try compiling all your code. I believe you'll get a compilation error. Correct any errors, clear your code coverage, then run all tests. After this, you should be able to calculate your code coverage.
You should go to developer console and choose Test->Run all, once the tests run you will have coverage displayed.