Word macro error SortIndexEntries( sub or function not defined

Question

I have the following Word 2019 macro code:

Sub CreateBibleIndex()
  Dim bibleRefs() As String
  Dim indexEntries() As String
  Dim i As Long, j As Long
  
  ' Regular expression pattern to match Bible references
  Dim regexPattern As String
  regexPattern = "([1-3]?\s?[A-Z][a-z]*\.)(\s\d{1,3}[:]\d{1,3}(-\d{1,3})?)?"
  
  ' Canonical list of books of the Bible
  Dim bookList() As String
  bookList = Split("Genesis,Exodus,Leviticus,Numbers,Deuteronomy,Joshua,Judges,Ruth,1 Samuel,2 Samuel,1 Kings,2 Kings,1 Chronicles,2 Chronicles,Ezra,Nehemiah,Esther,Job,Psalms,Proverbs,Ecclesiastes,Song of Solomon,Isaiah,Jeremiah,Lamentations,Ezekiel,Daniel,Hosea,Joel,Amos,Obadiah,Jonah,Micah,Nahum,Habakkuk,Zephaniah,Haggai,Zechariah,Malachi,Matthew,Mark,Luke,John,Acts,Romans,1 Corinthians,2 Corinthians,Galatians,Ephesians,Philippians,Colossians,1 Thessalonians,2 Thessalonians,1 Timothy,2 Timothy,Titus,Philemon,Hebrews,James,1 Peter,2 Peter,1 John,2 John,3 John,Jude,Revelation", ",")
  
  ' Find all Bible references in the document
  With ActiveDocument.range.Find
    .ClearFormatting
    .MatchWildcards = True
    .Text = regexPattern
    .Execute
    While .Found
      ReDim Preserve bibleRefs(i)
      bibleRefs(i) = .Text
      i = i + 1
      .Execute
    Wend
  End With
  
  ' Loop through Bible references and create index entries
  For i = LBound(bibleRefs) To UBound(bibleRefs)
    Dim ref As String
    ref = bibleRefs(i)
    Dim book As String
    Dim chapter As String
    Dim verse As String
    book = Left(ref, InStr(ref, " ") - 1)
    chapter = Mid(ref, InStr(ref, " ") + 1, InStr(ref, ":") - InStr(ref, " ") - 1)
    verse = Mid(ref, InStr(ref, ":") + 1)
    Dim indexEntry As String
    indexEntry = book & " " & chapter & ":" & verse
    If Not IsInArray(indexEntry, indexEntries) Then
      ReDim Preserve indexEntries(j)
      indexEntries(j) = indexEntry
      j = j + 1
    End If
  Next i
  
  ' Sort index entries in canonical order
  Dim sortedEntries() As String
  sortedEntries = SortIndexEntries(indexEntries, bookList)
  
  ' Create index in document
  Dim indexText As String
  indexText = "Index" & vbCrLf
  For i = LBound(sortedEntries) To UBound(sortedEntries)
    indexText = indexText & sortedEntries(i) & vbTab & "p. " & GetPageNumber(sortedEntries(i)) & vbCrLf
  Next i
  ActiveDocument.range.InsertAfter indexText
End Sub

Function IsInArray(stringToBeFound As String, arr As Variant) As Boolean

I get the error as in the heading. Cannot see how to fix. Can anyone help? Thank you