Hi @Chris Jacobs ,
As I know, this error may occur when you haven't configured the certificate successfully or not chosen the correct certificate in Outlook. So it's recommended to first check if you have chosen the correct certificate(File>Options>Trust center>Trust center settings>Email security )
If you have checked that, due to I could work with my certificate in Outlook and couldn't test on same certificate with yours, I have researched a lot about it and found a similar thread that some users said that solution could work for them, please kindly check if it could also be helpful to you, for your reference: Your Digital Id Name Cannot Be Found By The Underlying Security
If your issue has any update, please feel free to post back.
If the response is helpful, please click "Accept Answer" and upvote it.
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.