Can't open docx document with Pages 15.4

There are usually, and rarely, the following reasons why a Word docx will not open in any version of Pages:

• The Word document contains too many unsupported features that Pages cannot translate and it gives up.
• The Word document has become slightly corrupted, but fixable.
• The Word document isn't really a Word document, but a renamed document that Pages was never intended to open.

In the first instance, there is nothing you can do other than have the originator regenerate the Word document and resend it to you.

In the second, I and others have found that opening the Word document with the free LibreOffice Suite¹ and then saving it as a Word 2010-365 docx can often result in Pages successfully opening and translating the document. LibreOffice is a clone of Microsoft Office and derives from development dating back to the early 1990's (StarOffice).

¹ LibreOffice 26.8.2 (or later) is available in Intel or Apple Silicon downloads. Simple to install, and then open the Word document.

In addition to VikingOSX’s suggestions, you could try to open the Word document on the online version of Microsoft Office: https://www.microsoft.com/en-us/microsoft-365/free-office-online-for-the-web.