Typically, this indicates that the system is capable of running the chosen test, but that the device is not configured correctly or there is an issue with the device or the device driver that is preventing PC-Doctor Service Center from running diagnostics.
A good example of this is the "USB Ports Test". This test verifies any inserted USB devices can be recognized and is operating at the highest speed possible. This test requires a USB device, such as a USB flash drive, to be connected to the system before the test is run. If no USB devices are connected to the system, Service Center will report that the test "Cannot Run".