Diagnosis
That 'ERR' code is the grill's way of saying it can't reach or maintain the set temperature. For the Tailgater 20, this is almost always a problem with the flow or ignition of pellets in the firepot.
How to Fix It
First, power the grill completely off. Let's clear the firepot. 1) Unplug it and let it cool. 2) Open the grill and remove the grates and drip tray to access the firepot (that small cup at the bottom). 3) Use a shop vac or the Traeger-approved vacuum to suck out all the ash and any unburned pellet fragments from the pot. 4) Make sure the holes in the pot aren't clogged. Plug it back in, start it up on 'Smoke' with the lid open, and listen for the fan and the sound of pellets dropping. You should see ignition within 5-10 minutes.
If That Didn't Work
If you cleaned it and it still throws an ERR, the issue might be with the hot rod igniter. It's a small heating element that lights the pellets. Over time, they can fail. You can check it by starting a cycle, waiting a minute, then (carefully) looking into the firepot to see if the rod is glowing red-hot. If it's not glowing, it's likely dead and needs replacement.
Heads Up
Always start your grill with the lid open. Starting it closed can cause a dangerous over-pressure situation that blows pellets and ash everywhere and almost guarantees an ERR code. Lid open until it's clearly smoking and up to temp.