Unexpected [child element...] / expected: {}
This indicates that while the IRS or state's e-file processor was analyzing the e-file, it found an entry that it didn't expect to see until later on in the e-file, until after it had already seen other items - specifically, whatever it lists as "expected."
This means the 'expected' item's missing.
Both the expected and unexpected items will have field descriptions which should hint as to what place on the form the rejection's referring.
Use the unexpected item(s) to establish context as to where the expected item(s) should have been, and attempt to identify the line described as 'expected' and identify what might be missing about that item.
Many times, this refers to the system seeing an amount for a subschedule line item, without a corresponding description, or vice versa. This may include entering a zero.