Bulb knows that the previous occupant has moved out, but they don’t know you’re moving in. They need to know who you are, the date you became responsible for the energy bills and the meter readings on that day.
That bill is addressed to ‘The Occupier’ because they assume someone is living in the property but they don’t know who.
You only have to pay for energy used after you became legally responsible for the property - either by buying it or the start date on your rental agreement.
If it’s a rented property then the landlord is responsible for any energy used between tenants.
Tell Bulb when you moved in, then make sure when they bill you that they’re using that date, correct readings and it’s addressed to you. ‘The Occupier’ bill may still keep coming because of their inadequate system, but it’s not yours to pay.