In "generate" mode the System Generator token is "hidden", so if an error occurs that could be corrected on the System Generator token (e.g., change clocking from DCM to Clock Enables), the user double-clicks on the token in vain.
The only solution is to exit MATLAB, double-click on the model in the Vivado tool to launch System Generator again, and then change the setting.
For System Generator for DSP Release Notes from other versions, see
(Xilinx Answer 29595).