"Each Student SSN that has a value on form 8863, Line 21 must not be the same as that in another Form 8863 for the same tax year". What does that mean and what to do?

That error message is saying that another tax return has been filed claiming an education credit for the same student as shown on your return.

In order to fix this on your tax return so that you can electronically file it, you must remove the education credit that is being claimed. 

Or, you can leave the credit on your return and file it by mail.  Then, when the IRS is processing your return they will examine both your return and the one that is claiming the same student to determine which one was filed correctly to claim the credit.