Start by verifying that the drawing file you are trying to open was not created in a newer version of AutoCAD, with a newer file format than the version you are currently using. For example, a drawing created in AutoCAD 2015 will not open in AutoCAD 2012. Refer to AutoCAD drawing file formats for specific details on the…
Did a quick Google search: https://knowledge.autodesk.com/support/autocad/troubleshooting/caas/sfdcarticles/sfdcarticles/Drawing-file-is-not-valid-error-message.html Seems related to AutoCAD versions, are using a older version than the file? Or the file is corrupted?