Altium Error: Multiple top level documents has been used
In Altium you can only have one top schematic sheet.

From Connectivity and Multi-Sheet Design:


  While the project file links the various source documents into a
  single project, the document-to-document and net connective
  relationships are defined by information in the documents themselves.
  A multi-sheet design project is arranged as a hierarchical structure
  of logical blocks, where each block can be either a schematic sheet or
  a HDL file (VHDL or Verilog). At the head of this structure is a
  single master schematic sheet, more commonly referred to as the
  project's top sheet.


You'll have to make another project containing the Power Supply schematic and PCB documents.