When performing an iOS Full Migration, Mobix might ask for the applied iTunes encryption password, this is a password the user created when he performed an iTunes backup of this device.
In those cases, Mobix must use that same password the user had applied in order to migrate data as otherwise the data is not accessible.
In case the user does not remember the assigned backup password, follow apple's instructions (https://support.apple.com/en-us/HT205220) on resetting it and try again.
Note, iOS device with unknown applied encryption can still transfer Contacts, Images and Videos using the selective content transfer without inputting the iTunes backup encryption password.