I believe that error means the script was encoded with the SourceGuardian PHP5 encoder and thus will only run on PHP5, not PHP4. The script author will be able to tell you if that's the case
Make sure, first off that you have the proper SourceGuardian loaders for the version of PHP you're using. This might, also be the issue
If THAT doesn't resolve it, then you'll need to contact the vendor for an updated script
WHMCS Guru - WHMCS addons, management, support and more. WHMCS Notifications Extended - Add slack, hipchat, SMS, pushover to WHMCS !!
Linux Problems? WHMCS Issues? +1-866-546-8914 (linux-14) or @whmcsguru on twitter!