Here in Helaman 6:21, the original text had the preposition in at both the beginning and the end of the whatsoever-clause: “in whatsoever difficult circumstances they should be placed in”. Such repetition is considered unacceptable in standard English; thus the clause-final in was removed in the 1837 edition, and it has never been restored in any subsequent edition. As explained under Alma 23:1, the original Book of Mormon text had a number of whatsoever- clauses for which an initial preposition in was repeated at the end of the clause; some of these have been maintained in the current text. In two cases, the text will be emended by adding a repeated in to the end of such a whatsoever-clause.
Summary: Restore in Helaman 6:21 the original in that ended the whatsoever-clause, thus providing an in at both the beginning and the end of the clause: “in whatsoever difficult circumstances they should be placed in”.