this happens if you have a group without a UI element, for ex. nothing, or hidden, or calculate only in the group.
Here an example:
EmptyGroup01.xlsx (9.9 KB)
You can preview (and save) the form, but deploy gives the error:
You get the same error message, if you check your XLSFom with https://opendatakit.org/xlsform/
You have the same problem, if you only put KoBo metadata in a group, see example
EmptyGroup02.xlsx (10.1 KB)
A note, as single item in a group will work without error.
If the begin_group… end_group is not balanced, you get a different error message, even on preview already