I was seeing the same issue despite COM references being set up correctly, blowing away/redownloading dll file, etc. Then I found a similar issue online that outlined a way to uninstall/reinstall Office 365. Following these steps resolved the issue for me:
Open your Control Panel (Start Button on Desktop, then click the Settings gear icon), the select the Apps category and scroll down until you see 'Microsoft 365 Apps'. Click once on Microsoft 365 Apps and then select "Modify". I recommend a "Online Repair".