This question tells me you should start by understanding exposure first. Start with reading about the Exposure-Triangle. If you understand that, you would not be asking this :)

Briefly, exposure is determined by 3 parameters: ISO, Shutter-Speed and Aperture. When you are in manual mode and set all these, that is it. No further adjust is possible or needed.

When you are in an automatic mode, you adjust 0, 1 or 2 of these three and the camera determines the rest. The point is that there is at least one left. Exposure-Compensation shifts how the camera sets the parameters it controls. If it cannot control anything as in manual mode, there is nothing to shift.

Most mid-range cameras with manual-controls and a single control-dial use this to their advantage where the EC button switches between controlling aperture and shutter-speed in Manual mode. If your camera has dual control-dials, EC either does nothing or shifts the Exposure-Meter which can be used as a guide to set Manual exposure. It does not affect exposure in this case either.