I cannot collaborate in SharePoint on a Word document

To be able to collaborate on a Word document with multiple users, the format of the document must be .docx

If the document was create with a previous version (Word 2003 -2007), you must convert it in the new file format (Word 2010 -2013).

How to save a document on the latest version?

1. Open the Word document
2. Click on File > Save as
3. Select for Save as type = Word document (*.docx)