No external mode support for the current real time workshop target

I've installed raspberry pi support package for matlab and simulink , but when i tried to run the image inversion example on external mode i get this error No external mode support for the current Real-Time Workshop target.

Error occurred while executing External Mode MEX-file 'noextcomm': No external mode support for the current Real-Time Workshop target.

Unable to connect to the 'Raspberry Pi' target for 'raspberrypi_inversion'.

however I tried the same model but this time using normal mode, connecting the raspberry pi to a display unit,I deployed it and the model worked fine.

What could be the possible causes for this error ? Also I have followed all the instructions regarding the compiler and mex setup.