You can also install the extension manually.

To install the extension-

1. Log in the FTP, and then go to Magento 2 root folder (generally present under the public_html folder).
2. Create a new folder named code under the app folder; under the code folder, create a folder named Ced.
   • Upload or Drag and Drop app/code/Ced/Rakuten directory.
   • After successfully uploading the directory, the extension will be installed/upgraded.
   • Now run the following upgrade command in cmd
     

     php bin/magento setup:upgrade
     php bin/magento setup:di:compile
     php bin/magento setup:static-content:deploy -f
     php bin/magento indexer:reindex
     php bin/magento cache:flush

   • Once the extension is installed it appears in the main menu of the Magento panel