There are two likely causes for this warning message:
Antenna calculations are not yet performed, so the antenna data does not exist. Calculations for antenna measurements and annotations are only performed when needed. To plot antenna measurements or annotations, you must add the measurements or annotations before simulation occurs. Otherwise, the antenna data does not exist in the data set, and an additional simulation is needed. To resimulate after adding antenna measurements, right-click the EM Structure and choose, then simulate the structure again.
The antenna was simulated with AFS enabled. Because only a subset of frequencies are simulated with AFS, antenna data is only available for those simulated frequencies, and not the entire frequency list. In this case, you must use FSAMP to specify the frequency sweep of the measurement or annotation. See “Plotting Currents and Antenna Measurements with AFS ” for more information on using AFS and FSAMP. Otherwise, a resimulation with AFS disabled is required to plot antenna measurements at all frequencies.