The source application is busy and can't respond immediately.
This message sometimes appears when PowerPoint attempts to open an embedded object that is already open in another program, but that other program has generated a dialog box or error message box that must be closed. Adding insult to injury, clicking Cancel in the PowerPoint error message box just causes the same error to appear again and again because the software continually tries to open the inaccessible embedded object.
The open dialog box or error message in the other application must be closed before PowerPoint can open the embedded object. Click the program's entry on the Taskbar to bring it to the forefront, and click OK or Cancel to close the dialog box or error message associated with that program. If the Taskbar trick doesn't work, hold down the ALT key and tap the TAB key to cycle through open programs until you reach the one with the dialog box or error message, and proceed from there. Now, switch back to PowerPoint and it should have no trouble opening the embedded object.