I have a code saying that my auxillary battery is bad or not charging?

Hello, a multimeter is not a confident way of testing a battery. 12.8 volts tells us that the battery is charged, but not that it can handle the load of starting a vehicle. You can remove and take the battery to any parts store to have it tested, or of course drive the vehicle to those stores. As long as the battery is easily accessible for them they can test it in the vehicle.

William

It's a brand new battery

And I had them test it before I purchased it, I think it might be the connection between the relay and the ECU?

It's not uncommon for a new battery to fail a tes. If you can clear the code see how long it takes for it to return after clearing.

William