Hello -
The adapter you purchased is for Thunderbolt 1 and 2 devices, such as the 2011-2015 MacBooks and some desktop Macs of the same vintage. These connectors share their design with Mini DisplayPort ports and cables, but carry more data.
Your laptop has Thunderbolt 3 ports, which share their form factor with USB Type C (a smaller connector). Unfortunately without another adapter (Apple makes a Thunderbolt 2-3 adapter) you won't be able to use the adapter you purchased.
The good news is you can also get a Thunderbolt 3 to Ethernet adapter, just not from Apple. Belkin sells one through Best Buy that also has USB-C power pass-through.