mac not saving docx. files

Hi,

I am having trouble saving docx. files on my mac.

I tried to save an existing file, and the 'Save As' window popped up and asked me to select a location to save the document - but the document had already been saved in a folder. I re-entered the file name and clicked 'Save', but the window re-appeared after closing momentarily. The file will not save as a docx. and the 'Save As' window would continue to pop up. However, I was able to save the file as a pdf, as well as a Word 97-2004 document (doc.), but not as a (docx.).

I have not tried re-starting my computer in case my file does not auto-save.

How can I fix this problem and save my documents as docx.?

My mac is High Sierra version 10.13.1 with a 2.7 GHz Intel Core i5 processor and memory of 8GN 1867 MHz DDR3.