The issue with your code is that Find only searches for the complete text at once. Here's an improved macro that iterates through lines and checks for the code:
Sub Head6_To_Heading()
Dim doc As Word.Document
Dim rng As Word.Range
Dim para As Word.Paragraph
Dim foundText As String
Set doc = ActiveDocument
foundText = "(head-" ' Code to search for
For Each para In doc.Paragraphs
' Check if line contains the code
If InStr(para.Text, foundText) > 0 Then
Set rng = para.Range
' Apply Heading 6 style
rng.Style = doc.Styles("Heading 6")
Exit For ' Stop after finding the first occurrence (modify if needed)
End If
Next para
End Sub