When setting the values for damping ratios and periods for Rayleigh damping, you might get the warning shown in the picture below before starting the simulation;

This happens because Ashes is trying to compute values for the stiffness and/or mass proportional coefficients for the Rayleigh damping, but one or both these coefficients are negative. These two coefficients are computed based on the values for damping ratio and period entered by the user in the Analysis tab of the Analysis parameters window, so in order to fix this warning you need to modify these values.

You can find the values of the stiffness and mass proportional coefficients in the Model Information dialog, and you can see how the coefficients are computed in the How to use Rayleigh damping document.

Note: this message is a warning (and not an error), which means that the simulation can be run despite the issue. In that case, the problematic coefficent(s) will be set to 0 and the simulation will be run without damping