LyX ⇒ Activating APA style

[Ooohh_Shiny]

Hi - new user, here - please be gentle

I would like to use APA style. I searched the Docs > Settings and see there's an apa6 option in the Document Class menu which shows as "Unavailable: article (APA6). This gives me an error message with the following:

"... but the document cannot be compiled until the following prerequisites are installed:"
apa6.cls
apacite.sty

I found a post this forum suggesting to install the apacite package by installing texlive-bibtex-extra. I tried this, but there's no difference; the error is still displayed.

Could someone please assist me to locate and install the necessary packages?

Here's my basic system info:
* Linuxmint 17.3,
* LyX 2.0.8.1

Please let me know if you require additional information.

Thank you.