Hi, I'm John! I will help you with this.
Just to clarify, does nothing happen when you click Justify, or does the text stay left-aligned even though the Justify button appears selected?
A common cause is that the document contains a paragraph mark (Enter) at the end of every line instead of normal paragraphs. When that happens, Word cannot fully justify the text because each line is treated as a separate paragraph.
As a quick test, could you open the same document in Word for the web and see if Justify works there? This will help determine whether the issue is with the document itself or with the desktop Word app.
Also, please press Ctrl + Shift + 8 to show paragraph marks (¶) and let us know if you see a paragraph mark at the end of every line or any manual line breaks. Those can prevent justification from working as expected.