The Communications Toolbox requires both the DSP System Toolbox and Signal Processing Toolbox to be installed, as well as MATLAB. The function "isfi" is shipped with the DSP System Toolbox which needs to be installed for the correct functioning of the "qammod" function. As the "qammod" function calls the "isfi" function, if the DSP System Toolbox is not installed then "qammod" will produce this error message.
To resolve this issue please reinstall the Communications Toolbox along with the previously stated required system products for the Communications Toolbox. This should resolve the issue.
I have attached a link to the Product Requirements for the Communications Toolbox below:
