What is meant by needs to be vented?can this be installed on the perfected paper over one layer of shingles? And not void warranty?

If you are installing over shingles and there is an air space between the existing roof and the new metal panel roof, condensation will form under the new metal roof due to the warmer air below the panel. That area will need to be vented so that the air gap below the panel is the same temperature as the air above. If you can't vent the space another option is to fill the space with new insulation.