The ACX Check plugin is a “Nyquist plug-in”.
The latest version is here: http://wiki.audacityteam.org/wiki/Nyquist_Analyze_Plug-ins#ACX_Check
This version replaces older versions which do not run properly on recent versions of Audacity.
Installation instructions for Nyquist plug-ins on Windows: https://manual.audacityteam.org/man/installing_effect_generator_and_analyzer_plug_ins_on_windows.html#nyquist_install