Cable?

Most external SSDs are using the USB protocol which is not compatible with Thunderbolt 2. If the SSD you have utilizes the USB protocol, then you will need to connect it to a USB port (preferably a USB3 port....if the SSD only has a USB-C connector you can use an Apple USB-C to USB-A Adapter). If the SSD utilizes Thunderbolt protocols and has a USB-C connector on the drive, then you can use an Apple USB-C to Thunderbolt 2 Adapter.

You first need to confirm the protocol that the SSD supports. Some drives may show an icon, but if no icon is next to the ports on the SSD, then check the SSD's documentation. Generally I assume most drives will be using the USB protocol because most people don't want to pay more for a device with Thunderbolt support. USB icon looks like a trident, whereas Thunderbolt has a lightning bolt icon.

There is no 2016 Mac mini model.

If it is a 2012 or 2014 model, then it has USB-A 3.0 ports.

If it is a 2018 model, then it has USB-A 3.0 ports and the faster USB-C ports.

What year model is your Mac mini?

see > Identify your Mac mini model - Apple Support