Eagle PCB: "No Supply for Power Pin ..." error
The Electrical Rule Check executes, depending on the pin direction (specified by the author of the schematic symbol), various checks. It expects for the direction of type Pwr a Sup pin set for this net. Sup is a pin type for power supply outputs for ground and supply symbols.

From the Eagle manual:


  "[...] For every Pwr-pin there must be at least one pin with the same name but the
  direction  Sup(a supply pin). There must be one on every sheet. These Sup
  pins are fetched into the schematic in the form of power supply symbols, and
  are defined as Devices in a library (see  supply*.lbr). These Devices do not
  have a Package, since they do not represent components. [...]"


Further...


  "If there is no supply pin in the supply libraries that fits to your voltage in
  the schematic, you have to define a new supply pin! Renaming an already
  existing supply pin is the wrong way and can lead to unexpected results!"