A demand pressure can only be set at exit points from the system, i.e. the boundaries or end nodes.


A demand pressure, referred to as an End Pressure in the Node pane, is the required pressure demand at the end of a pipe when the fluid is discharged. For example, in a sprinkler system, each sprinkler head may have a required minimum pressure. To find the fluid flow rate exiting the pipe system, a demand pressure is drawn and the required pressure set for each sprinkler head. Demand pressures can only be set at the end of a pipe that does not join into any other pipes. 


A pipe can have a demand flow or a demand pressure at the end of a pipe but it cannot have both. 


In the real-world it is the difference in pressure between two points that determine the flow rate that will occur and hence this is why you cannot define a flow rate and a pressure condition at the same point. If you define a flow rate then the software will calculate what pressure is required at that point in order to get that flow rate when considering all of the other flows and pressures throughout the system. If you define a demand pressure then the software will calculate what flow rate will occur.