First, ensure that you are placing your finger correctly on the scanner plate on the reader.
If you are placing your finger correctly on the scanner plate, and still seeing only a red flashing light, there may be another problem.
Enrolment issues
Are you sure that the finger you are scanning has been enrolled in the system? Each fingerprint on each hand must be enrolled separately, so if you try to scan a finger that has not be registered, it will not be recognised.
If you are sure that this is not the case, try deleting the fingerprint out of the system. Then re-enrol the finger and try again.