Why does all my dashboard lights shut off when I turn on my headlights?

The dimmer switch is on. The dashboard lights work except when I turn on my headlights then its dark can't see speedometer

When the lights are on move the dimmer switch to both directions. If nothing happens you need to check the dimmer switch itself or trip reset/illumination switch. Which behind the cluster, then you have to check voltage and continuity from and to the combination meter. If you have some knowledge of electrical system go for it otherwise let it be.