Running macro to specific style - vba

Question

Below code is trying to convert words in lowercase in to uppercase. However I only need to run it only in a specific word style ("Normal"). I tried to set doc to ActiveDocument.Styles("Normal") but i keep on getting error. Any help would be most helpful. Thank you in advance.
Option Explicit
Public Sub TitleCaseDocument()
Dim doc As Document: Set doc = ActiveDocument.Styles("Normal")
Dim wrd As Range
For Each wrd In doc.Words
If wrd.Text <> UCase$(wrd.Text) Then wrd.Case = wdTitleWord
Next
End Sub

Answer 1

The solution provided by #eaazel falls into the default member trap.
The code
wrd.Style
is in reality using the default member of the style object, which is 'NameLocal'. Thus the code implied by the code above is in reality
wrd.Style.NameLocal
Normally this would not be a problem, however, the level of granularity that is being used to extract the style object means that, on occasion, words with no style will be encountered (e.g. a ToC field). In such a case the style object returned is nothing and this generates a surprising error because you cannot call the NameLocal method on an an object that is nothing.
Therefore a more correct approach is to use a word unit that is guaranteed to have a style object (e.g. paragraphs) and to test for the style on this object before testing each word.
Option Explicit
Public Sub TitleCaseDocument()
Dim myDoc As Document: Set myDoc = ActiveDocument
Dim myPara As Range
For Each myPara In myDoc.StoryRanges.Item(wdMainTextStory).Paragraphs
If myPara.Style.NameLocal = "Normal" Then
TitleParagraph myPara
End If
Next
End Sub
Public Sub TitleParagraph(ByVal ipRange As Word.Range)
Dim myText As Range
For Each myText In ipRange.Words
If Not UCase$(myText.Text) = myText.Text Then
myText.Words.Item(1).Case = wdTitleWord
End If
Next
End Sub
Update 2020-Apr-16 Revised code below which has been proved to work on a Word document.
Option Explicit
Public Sub TitleCaseDocument()
Dim myDoc As Document: Set myDoc = ActiveDocument
Dim myPara As Word.Paragraph
For Each myPara In myDoc.StoryRanges.Item(wdMainTextStory).Paragraphs
If myPara.Style.NameLocal = "Normal" Then
TitleParagraph myPara
End If
Next
End Sub
Public Sub TitleParagraph(ByVal ipPara As Word.Paragraph)
Dim myText As Range
For Each myText In ipPara.Range.Words
If Not UCase$(myText.Text) = myText.Text Then
myText.Words.Item(1).Case = wdTitleWord
End If
Next
End Sub

Answer 2

So Do You want to change lowercase in to uppercase if style is normal?
Yes?
I don't have big experience with word but maybe something like this help you (base on your code):
Public Sub TitleCaseDocument()
Dim doc As Document: Set doc = ActiveDocument
Dim wrd As Range
For Each wrd In doc.Words
If wrd.Text <> UCase$(wrd.Text) And wrd.Style = "Normal" Then
wrd.Text = UCase$(wrd.Text)
End If
Next
End Sub