You can open the Dynamic Error Budgeting tool in the 20-sim Simulator. This tool allows you to enter disturbances (as power spectral densities) at any point in your model. For each disturbance you simply have to select a corresponding model variable and define the noise properties. By selecting the output you can inspect the results of these disturbances at that specific output.
In the graph at the bottom right you can see the resulting error at the selected output as a result of the disturbances. The error is given in the form of a power spectral density (PSD) and cumulative power spectral density (CPS). The square root of the final value of the CPS is equal to the standard deviation of the output error. The standard deviations are shown in the Output Sigma tab.
The Dynamic Error Budgetting tool is part of the Frequency Domain feature of 20-sim.