This is normally due to the way the SCORM zip file was incorrectly created. If you allow Captivate to create the zip by just ticking the Zip option in the Publish dialog, then the process is performed correctly and the imsmanifest.xml file will be located at the root level inside the zip. However, many Cp authors try to create the zip file by selecting the FOLDER containing the SCORM files and then zipping the folder. Unfortunately, this means that the imsmanifest.xml file that the LMS needs to find is now sitting one level deeper in the structure than it should be.
Bottom line: Let Captivate create the zip file. Alternatively, navigate INSIDE the folder containing all of the project published SCORM files, select all files there, right-mouse-click on the selected files, and choose Winzip > Add to [foldername].zip. This will also create a zip with the imsmanifest.xml file at the root level.