My 14 y.o daughter has tension/cluster headache, went to the doctors twice & was told its due to bad posture&smartphone use. worried. second opinion?

Diagnosis?????: If that is a correct diagnosis she needs a neurologist. Your statement implies that the Drs arrived at a different diagnosis.

Types of headache: You mentioned two types of headache with different types of symptoms, tension headache symptoms are different from cluster headache. Agree that if she suffering a lot she needs to see a specialist in headache, a neurologist, meanwhile she has to observe good posturing while using computers/smartphones as what the doctor(s) suggested, use painkillers, unless contraindicated, good luck