When I set the resolution on 0.5nm and sampling interval on 0.5nm, I see the error of “Unsuitable Resolution”, what does it mean exactly? Does it mean my measurements are not accurate?

This means that the resolution setting is not appropriate for the span and the sampling number. Please see the attached image for the relationship between number of samples, span, and resolution. If you receive this error, it could mean that some data may not have been extracted completed from the sweep.