You can retake a subject test. 

Please note that each vendor has a different retake policy.  

HiSET subject tests can be taken up to 3 times per calendar year (January-December).

GED subject tests can be scheduled up to three times.  After the third attempt, a candidate must wait 60 days before scheduling again.  

The voucher codes provided by the Commonwealth of Massachusetts are valid for three attempts per subject.  "No Shows" count as an attempt.  After the third attempt, the candidate is responsible for payment.