It appears the system supports one M.2 SATA and one 2.5" SATA drive -- that said, you will need to remove the base cover to check inside.  Most drives require mounting hardware, and systems ship without the hardware needed to mount a drive not included with the original configuration - you will then find you need to order a bracket, cable and screws to mount a 2.5" SATA drive -- which you'll need to order from Dell Spare Parts.  
I already have an M.2 SSD can I add and use it with a SATA SSD? or HDD? It's a Dell Vostro 5568... Thank you!!
HARD DRIVE VOSTRO 5568