So this link is obviously not Indusoft but it is about Kepware and not being able to find a matching endpoint so maybe you can get some ideas from reading this:
Basically, check your endpoint URL and your security policy. If you change anything on the Kepware side, make sure you restart the service so the change will actually take.
If none of that works, consider switching to Ignition, where you have auto OPC discovery, useful log messages and an active forum of people who know Ignition 