Im not too sure about soapui pro, i use readyapi! and free soapui.
Depending on your app, there are multiple ways to create projects containing APIs....it all depends what type of service youre testing (SOAP? REST? etc.) and if you have these APIs defined in a definition file like a .wsdl, .wadl, swagger, openAPI, etc. or not.
If you havent any of these definition documents you can create an empty project and manually add a single request at a time to build APIs in your project.
I wouldnt normally expect the error response you mentioned to be generated from just adding APIs into a project. The error response implies some sort of connection request failure.
if this helped answer the post, could you please mark it as 'solved'? Also if you consider whether the title of your post is relevant? Perhaps if the post is solved, it might make sense to update the Subject header field of the post to something more descriptive? This will help people when searching for problems. Ta