Hi Steven,
To better understand your situation, I'd like to confirm some more detailed information with you.
1.Your Exchange server version.
2.A screenshot of the cmdlet you run and the complete error message you got in Exchange Management Shell.
A SSL certificate is required to be bound to IIS in an Exchange server and its SAN must include the server's FQDN. In the Default Web Site, the certificate is bound for HTTPS Port 443. In Exchange 2013 and Exchange 2016, a certificate that matches server's
FQDN is also required to be bound to the Exchange Back End website for HTTPS Port 444. It is suggested the Exchange Back End certificate bound for 444 always is configured to use the self-signed certificate.
Please let me know 3: if there is any certificate that can be selected in the SSL Certificate field. If no, you can create a self-signed certificate for the configuration in Exchange admin center referring to
https://technet.microsoft.com/en-us/library/dd351057(v=exchg.160).aspx.

Moreover, please also check the virtual directory settings especially for PowerShell. The default settings are covered in this blog:
https://blogs.msdn.microsoft.com/mvpawardprogram/2013/03/18/virtual-directories-exchange-2013/. If there is any misconfiguration, you can change it to the default.
Regards,
Alex Sun
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