Cannot find X.509 certificate using FindByThumbprint

Maybe you don't have the certificate installed on that machine. Take a look on Internet Explorer (Tools | Internet Options | Content tab | Certificates) installed client certificates.

Under "Personal" tab you can see the installed certificates for you user account (as you are using the "My" store name). Double-clicking on an installed certificate under Details tab you have all information you can use on find value attribute, like the thumbprint.

Note that on Windows 10, the certificates are found in the local "Computer" store. On Windows Server 2008 R2 and below, they will be found in "Personal".