There seems to be an error with your plugin, I could not delete the coupon once it is applied, I use the latest version. Here is the link: https://www.peilishop.com/cart/, you can try this coupon “qjab74fh”
It looks like an incompatibility with your theme (Minimog) and the ‘Allow applying when invalid’-option. Can you untick the ‘Allow applying when invalid’ and try again, please?