Hi there, Charlie.
I see that you're being asked to supply the 6-digit passcode of your old iPhone. The problem is, your old iPhone passcode was only 4 digits long. I'm glad to help with this.
Use a passcode with your iPhone, iPad, or iPod touch -- Try entering your 4-digit passcode and see if it's approved. If not, try using the 6-digit code you just set up. If you have another device that signed into iCloud, and it uses a 6-digit passcode, try entering that passcode to see if it's accepted. If you don't have another device, or the passcode isn't accepted, choose "Can't Approve" option to reset the data and refer to the information below:
iCloud security overview -- This article provides the information regarding the data that won't be approved under "End-to-end encrypted data".
Take care.