If you are having issues opening encrypted emails in the installed Outlook client on your PC, you can try opening them from Webmail:  https://outlook.office365.com.  If you can successfully open the encrypted emails within Webmail, below are the troubleshooting steps you can try to resolve the issue.  Otherwise, if you cannot open the encrypted emails within Webmail, the issue is likely on the sender's end, and you will need to contact the sender to resolve the issue.

Generally, if you are having issues opening encrypted emails in the installed Outlook client, signing out of your Office365 account and signing back in will resolve the issue.  Please try this by following the steps below:

1. In the main top-right Outlook window, click "File"
2. In the bottom-left File menu page of Outlook, click "Office Account"
3. Under User Information, click "Sign out"
4. If you receive the "Sign out of Office" dialog box, click "Sign out"
5. Close Outlook and reopen Outlook
6. In Outlook window, click "File"
7. Click "Office Account"
8. Click "Sign in"
9. Follow prompts to Sign in
10. You may need to close and reopen Outlook again if it still shows "View Only (Unlicensed)" in the Outlook Account page after signing in