This error indicates an DNS look up failed at some point during the test, default behavior is to abort the test immediately. If you wish to continue your test even in the case of failed DNS resolution please enable the 'DNS Continuation' option which will fail these transactions but not stop/abort the entire test. (applies to versions 4.xx)
Please note previous releases (2.xx - 3.7x) for HTTP action lists which require DNS resolution on the domain name must have 100% successful resolution or the test will abort without results.