Unable to transfer data due to old iphone 6 battery flat

I had started to transfer data from iphone 6 to iphone 12 pro and my old phone battery went flat interrupting the connection. When I found the charger and the old phone charged the new mobile told me to move closer, but then it changed to unable to transfer data due to loss of connection.

Now when I try to reset the transfer, it tells me it will delete all data and restore factory settings...

How do I reconnect the transfer of data from the old phone please?

(to add to this when I bought the iphone 12 they forgot to sell me the charger!)