Lightroom will show the "absolute" dial for white blance when the file being edited is a RAW file.

When the file is an already processed format (JPEG or TIFF) it will show the "Relative" slider.

The reason for that is that having a raw file means nothing (or very, very little) hasbeen processed, thus a White balance can be arbitrarily choosen.

In a JPEG file (or similar), the image has already been processed, and one of the variables, the white balance, was already choosen and is impossible or very, very difficult to know (numerically) what was the actual color cast of the light that entered the camera. The only thing that can be done is to shift the color values of the pixels relative to what they currently are, but without knowing what the original values captured by the sensor, there is no point in showing absolute values.

I recommend learning more about color temperature and white balance, to better know what a white balance number means on the slider. The short version is that around 5000-5600 the light is considered to be white (equal proportion of all wavelengts, i.e. colors, are present).

Any other values means the ligt has a color cast, that is, there is a greater proportion of some colors. Higher values mean more blue, lower values means more red.

When a camera produces a JPEG file, it is simply processing theraw data from the sensor, applying a white balance transformatio, a color profile and other parameters to produce a "finalized" product, While a RAW file is the base data as seen by the sensor, and most of these parameters are yet to be applied, in this case, by Lightroom to produce the final product.