Hello.

The OPC UA server does not trust the client. You need to explicitly tell the server to trust your client. How it is done, is server specific - consult the server documentation. Many servers will have some user interface to manage the trusted certificates. In such you will likely find your client's certificate under Rejected certificates, and need to perform some action to trust it.

It looks like you are using KepServerEX. In this case, switch to the Trusted Clients tab in the OPC UA Configuration manager, and handle it there.

Also, the server log should contain an explanation of why it does not accept the client certificate (but if you have not explicitly made it trusted, the reason is clear).

Unrelated to this problem: Looking at your code, you are setting the SecurityPolicyUriString and AllowedMessageSecurityModes. This is OK to do if you want to assure precisely these settings. But in this specific case, if you were not setting them, nothing would change - because the way you configured your server