You have to expose the transaction as a webservice inorder to execute it from the webservice Action block.
Try this,
http://<servername>/XMII/WSDLGen/<TransactionFolder(s)>/<Transaction>. For example, if you have a transaction named Line1OEE in a KPI folder, and your server's name is Atlantis, the URI is http://Atlantis/XMII/WSDLGen/KPI/Line1OEE.
Web Service Interface - SAP Documentation
Incase you are calling any other URL, make sure its a webservice. When you put the same URL in the browser, it should return you a WSDL.