Hi Hung,

I see you have overpassing sides in your example so I use a similar one 

you need to point your cursor in the lower volume were you need the plinth added. It will light up in red 

right click and choose add plinth

You can set the plinths you need ( front and back ) with eventual recess

When you have no overpassing sides you need to add an elevation plinth height

this will create the lower volume so that you can add plinths

Be informed that you can only add side plinths when your side panels are underpassing

regards

Michel