Which discount plugin? What version of nopCommerce?
Howe did you install the plugin? (did you use the Local Plugins page with "Upload plugin or theme" followed by "Install" on the plugin line, or did you do some type of manual install?)
nopCommerce version : 4.40.1 (source code from Github). Plugin : the one included by default, in the local plugins page and "Discount requirements" group.