Air is not a pure substance. Air is a mixture of gases, mainly nitrogen (78.09%), and oxygen, but containing much smaller amounts of water vapor, argon, and carbon dioxide and very small amounts of other gases. Anything that is a mixture' is not a pure substance.
Air is a mixture and not a compound because air can be separated into its constituents such as oxygen, nitrogen etc. by fractional distillation of liquid air. Air shows the properties of all the gases present in it.
Whereas a compound is a chemical substance that forms when two or more different elements combine in a fixed ratio. Water ( H 2 O ),