Help in designing filter for DUC (Digital Up-Converter) and DDC (Digital Down-Converter) using the built-in DUC and DDC blocks in MATLAB Simulink
Help design my system would be far more detail than what we typically provide here, but I can give you some suggestions that may help or at least lead to a more concise question. 


  I do not get correct BER, which means my signal is getting aliased as
  I receive through the DDC.


Not getting the correct BER does not necessarily conclude this. 

In your case the error rate appears to be 50% which suggests your receive data is completely independent of your transmitted data - one possible reason this is if the delay for all the filtering hasn't been properly accounted for when comparing the transmit to the receive signals. Instead of comparing the constellations, consider comparing the baseband modulated signals in the time domain as waveform amplitude versus time to help determine what the actual failure mechanism is. That will show you if everything is fine and you just have the expected time offset, or otherwise give you further visibility into where exactly the problem is occurring (look at the in/out of each block in the time domain).

Once you isolate the block that is not performing as you would expect it to, it will either be quite clear to you what the problem is or you will have a more concise question to ask here.

Good luck!