2018 Vauxhall Astra rear indicator /break light intermittent fault

Check / replace bulbs , check wiring plug , check printed circuit panel . Intermittent faults are often not easy to find

Thank you for your advice
Can you tell me how to check printed circuit board

Just check you are getting power at bulb contacts , and no obvious breaks but usually should be fine with that year of car . On older cars you can sometimes get water ingress / damp causing faults