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Hi May,

Have you installed latest update to AutoCAD 2015?
Here is the link to Update:
https://knowledge.autodesk.com/support/autocad-for-mac/downloads/caas/downloads/content/autocad-2015...

If you already installed Update 4, I'd recommend you to perform clean uninstall:
https://knowledge.autodesk.com/support/autocad-for-mac/learn-explore/caas/sfdcarticles/sfdcarticles/...

Then install 2015 version again (if you want to stick with it) and install the Update I mentioned above.