I would like to ask: why as a new customer of Bulb I have to wait 16 days to sign in my old meter readings?
Although you may have joined Bulb today (for example) your switch wont actually take place until a further 21 days. During this 21 days your previous supplier is responsible for supplying your energy.
16 days after you joined Bulb you will be emailed by them with a request to supply your meter readings (this is 5 days before the actual switch). The meter reading should be those on the 16th day or as soon as possible afterwards and should not be your old meter readings.
These readings will be used by Bulb as your opening readings and will be supplied by Bulb to your previous supplier for them to work out your final bill.