I don't really understand what you are trying to do. You have never set a value for tagToFind. However, I see no reason to loop through all of the CCs three times to set a value:

Sub CheckBoxesByTag()
Dim propertyName As String
Dim propertyValue As String
Dim cb As ContentControl
Dim oCCs As ContentControls
Dim tagToFind As String
  'Get the value of the custom property
  On Error Resume Next
  propertyValue = ActiveDocument.CustomDocumentProperties(propertyName).Value
  On Error GoTo 0
  'Check if the custom property exists and has a specific value
  If propertyValue = tagToFind Then
    Set oCCs = ActiveDocument.SelectContentControlsByTag(tagToFind)
    For Each cb In oCCs
      With cb
        .LockContents = False
        cb.Checked = True
        .LockContents = True
      End With
    Next
  End If
End Sub
.