Connect Magic Trackpad 2 without cable?
Make sure the power is on to your trackpad. You should have a BlueTooth icon on your menu bar. Click on this icon and it should list all Bluetooth devices seen by your computer. Click on the Trackpad and then click on the connect button.
I had the same issue, what worked for me was checking the "Show Bluetooth in menu bar" in system preferences -> bluetooth. Once I checked that the magic trackpad showed up