Hi Thanya,
In Joomla, an article can only be assigned to one category. Joomla does not natively support assigning multiple categories to a single article.
If assigning multiple categories is a strict requirement, this can only be achieved using third-party extensions, as it is not supported by Joomla core.
Some extensions that can help include:
• FLEXIcontent (supports advanced category relationships)
• K2 (older, but supports multiple categories)
Please note that using extensions changes the default Joomla content structure and adds additional complexity.
In short: Multiple categories per article is not possible in Joomla core.
Kind regards,