This error can safely be ignored. You should have been able to successfully register the physical Workflow server and assign a Logical Workflow server to this physical server.
The error happens because physical servers are implemented as Integration Server remote server aliases, regardless if the physical server is a Workflow server or an IS server. When the WmAdmin server tries to verify the connection between the WmAdmin server and the remote WF server (usually by calling wm.server:ping on the remote server) it fails with the error you’re seeing, for the obvious reason that you can’t call the IS service wm.server:ping on a Workflow server.
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