Word doesn not use "et al." in citations using APA standard

Edit the citation style file from this folder: 

C:\Users\<yourusername>\AppData\Roaming\Microsoft\Bibliography\your preferred style

e.g., for apa style

C:\Users\<yourusername>\AppData\Roaming\Microsoft\Bibliography\

APASixthEditionOfficeOnline.xsl

Change to be made and follow same for other types:

1. Open the file in text editor. 
2. Find the section where it checks if there are more authors than allowed in citation (cAuthors > maxCitationAuthors).
3. Replace the line:
   <xsl:call-template name="templ_str_AndOthersUnCap"/>
   with this:
   

   <xsl:text>et al.</xsl:text>

4. Make sure this is set to 1 or to the number of author you want to show:
   <xsl:variable name="maxCitationAuthors" select="1"/>
5. Save and after this, citations appear like:
   Single Author: Sundar
   Multiple Author: Sundar et al.
   
   

Note: Make sure to add authors using firstname, middlename, lastname by adding one by one. Not custom separated with comma. 

restart the word, then see the changes.