Are you referring to exposure compensation where you've got a scale that looks something like this: -3, -2, -1, 0, 1, 2, 3 ~ which is a 7-stop wide scale?
There is only one possibility that I can think of where the camera would not allow you to go past a specific point on the scale. Let's say you have set your camera to manual exposure. You aperture is set to the maximum setting of f/22 and your shutter speed is set to 1/1,000. If you use a negative exposure compensation, your camera can do that up to 2 stops. In other words, you're already at 1/1,000th. If you set exposure compensation to -2, then the camera will use 1/4,000th which is two stops less exposure than 1/1,000th. However at 1/4,000th, the camera is now using the maximum shutter speed of the T6. But what if you want -3 stops of exposure compensation? The camera cannot close the lens down any further because you're at the minimum f/22 setting, and it can't use a faster shutter speed because you're already at the maximum 1/4,000th shutter speed. The camera simply cannot reduce the exposure by -3 stops. This is why you cannot choose -3 in this scenario. If you need to reduce the exposure more, in this scenario you would have to use a neutral density filter.
This would be the same if your lens was set to the maximum aperture, say f/2.8, and your shutter speed was set to 15 seconds. If you wanted +1 exposure compensation, the camera would use a 30 second exposure instead of 15 seconds. But the T6, like most cameras, do not have the ability to make an exposure longer than 30 seconds. In this case, the camera would not allow you to choose an exposure compensation setting of more than +1 because it cannot make an exposure longer than 30 seconds except when in bulb mode.