Cellphone is not working

I bought Sumsang A3 Galaxy Core on the 25 January 2022 at Ackerman's Muizenberg Capricorn. The cashier name is Zoe Hendrick she didn't activate the phone she only inserted SIM card. And I didn't take note of that because I was carrying my 1 week newborn baby. When I got home I switched on the phone the screen was saying "you can't use this phone without authorization" Today I returned the Manager said she can't help me because Zoe is off today I must till Tuesday and she was irritated I don't know what I must do because the phone is not working.