How can I use the buffer block in MATLAB Simulink for real-time signal analysis?

I want to process a multichannel signal in real-time using matlab simulink.

As far as I know in order to implement a frame-based processing (window-based or having a sliding window) I need to use a buffer.

Unfortunately there is an error while I want to run the model. It says that the sampling rate shouldn't be continuous or variable. But I have not changed the sampling rate explicitly. I have just chosen the buffer properties as this: buffer output size = 256, overlap size = 128. This is because of the stream signal sample rate (128 Hz). Maybe this choice makes it look varied.

I have also set the simulation solver options to discrete step, discrete solver and step size of 1/128.

Does anybody know how I can tackle this problem?

Thanks Regards, Reza