If you passing a folder path you need to pass it as a zip file. Folders are not accepted by mail directly. You can pass the file path directly in the actvitiy.
Use Compress/Zip files to zip the current folder and pass that in Send Outlook Mail Message activity.
Compressed file name: Give a name like Output.zip. A Zip file will be created within the project with name Output.
Content to zip: Give the folder path you want to zip. Select folder for that.
Compressed file: Create an variable out there and pass that variable in Send Outlook Mail Message.
Regards,