i can't do the back up of my old phone to transfer to the new one

Something in here might help.

If you can't back up to iCloud or restore from iCloud backup failed - Apple Support

However, I generally suggest force closing the settings app, restart your phone and then try again. If the iCloud backup is stuck, you can try and cancel it and then force close the settings app, but if it will not quit, just close the settings app, restart and try again.

If something else’s is going on, you need to provide more information. Simply stating that you can’t backup doesn’t give us any where to start with suggestions.