Does anyone know why this happens?
The higher I set the “Metallic” and “Smoothness” the darker it gets, the only way I can fix this is by adding a reflection probe.
With reflection:
No reflection:
This is the material:
The reflection probe was added to GameObject itself.
More metallenes and more smoothness will result in more influence from the environment reflection.
If that reflection is utterly dark (not set in the lighting tab to something useful its usually black) then the objects gets dark.
If you add a reflection probe / have a valid reflection probe influence at the objects position then the reflection should be properly bright.
Thank you 
Removed the reflection probe and changed the lighting
(Set “Sun” to “Direction Light (Light)” and lowered “Ambient Intensity” to 0)