I would like to apply a sinusoidal motion to a driver and thought this could be done with t (Main.Study.t) but I always get the above error. The error sounds logical although I thought this is the method of choice .
From the description you have provided, my understanding is that you are assigning time to a variable of type DesignVar.
However, for us to better understand and be able to address the source of the issue more precisely, I highly recommend you post the code snippet.
You cannot use a time variable function within the AnyKinEqSimpleDriver. If you want to use a sinusoidal function driver, you can use the AnyKinEqFourierDriver and set the cosine coefficients to zero.
Time-variable constraints on the activations can be done because they are at the dynamic level. In other words, βConstβ in your code snippet is of the VelVar type (evaluated at the velocity (dynamic) level) whereas the DriverPos was of the DesVar type (evaluated at the load time).