This is an issue that has appeared in the latest version of the plugin. We are working on the fix. Please downgrade the plugin to version 3.8.1. You can download it here: https://wpde.sk/po/18n3ybtczpss.png
After you install version 3.8.1, you will be able to move the invoices to the Trash.
I will let you know when the updated version will be ready for download.