What is the meaning of the notification "Valve cannot deliver flow", in reference to a Flow Control Valve (FCV)?
This means that the flow through the valve is less than the setting that you entered, so it is inactive (fully open). Note that an FCV does not set the flow; it limits flow by throttling (dynamically closing until the headloss is such that the flow is restricted down to the desired setting). Although the message appears as a warning (yellow) it does not indicate unstable results or necessarily a problem that needs to be addressed. It is just to warn you that the valve is not throttling.
If the flow through it is showing up as greater than the setting this could mean that the orientation of the valve is incorrect. Check the "downstream pipe" property of the FCV. This is due to the flow reported at the valve having a negative sign in front of it - an FCV will not limit negative flow, even though this just refers to the direction of flow.
If you are trying to use a FCV to control the flow from a source, you may want to consider modeling at least a skeletonized version of the network all the way back to the real source (pump and/or reservoir/tank) for the most accurate results.
Why do I see such a large headloss through my FCV, PSV or PRV?
Problems when using a flow control valve (FCV) downstream of a pump
How do I model a system where there is intermittent demand or where demand is being cut off from it's water source at some point?
Using a Flow Control Valve (FCV)