The System.EventHandler delegate expects you to have a method with a specific signature present. See:
EventHandler Delegate[
^]
So your code should probably be like:
protected void lnkCustomer_Click(object sender, System.EventArgs e)
{
...
}
Addition:
Casting the sender to link button:
protected void lnkCustomer_Click(object sender, System.EventArgs e)
{
LinkButton button = sender as LinkButton;
if (button.CommandName == "CustomerLink")
...
}