1. It's not an error, it doesn't stop anything even it's a bit annoying to keep looking at. But it's still better to specify. If you're going to use tif as a file type, then the 'short' and 'byte' data types can apply. If you need higher color range, then you can switch the file type to the commonly used 'exr' and the data type to rgba float or half.

2.In the Render View window, go Render>Render Selected Objects Only and uncheck it.