Timer Job Issues

This is the most common error in timer job.

Keep your scope as site

  public override void FeatureActivated(SPFeatureReceiverProperties properties)

  {

            SPSite site = properties.Feature.Parent as SPSite;

}

get the site as mentioned above.

If still you get the same error please check your code whether you are using any custom dll.
Because timer job sometimes will not accept custom dll files. so try to avoid those.