Assertion error: The current megafunction is configured for use with the clear box feature and cannot be used when the clear box feature is disabled

One way to fix such a clear box assertion error is to remove the AHDL Text Design Files (.tdf) from your project directory for the megafunction generating the error, and let the Quartus® II software select the TDF files from the current Quartus II installation directory.

A clear box assertion error can be generated when you use a newer version of the Quartus II software to compile an Quartus II project that was archived in an older version.  The older version may have TDF files that are not compatible with the newer version and you should remove these files before you start synthesis.