I simulated the model attached by you and it showed me four error that variables R1, R2, C1 and C2 are unrecognized variables. You can try fixing this error by either of the following methods:
a) Load the valueof four variables in base workspace in the command window directly.
I loaded value then and there and it worked fine, or else
b) In general you can make a file where you can define all the variables and then you can load that file before launching the simulation.
Both of these methods should work fine. For more information you can refer to the following document page:
Hope it helps !