in this website address https://www.xn--80ajjpgtb.xn--90ae/ i tried to install your add-on but the system says “Plugin not found” after uploading newsletter-extensions-1.4.1.zip file
Hi, that message usually means the package has not been unzipped correctly.
You can try to upload the newsletter-extensions folder inside the package via FTP inside the wp-content/plugins folder, then it should be listed and can be activated.