Hey -
When you say that you imported files, was this done while the project was on Windows or after copying to Linux? Can you right click the Content folder in the Content Browser and select the “Fix up Redirectors” options and try to package again? Additionally, if you’re already using the project on windows, you can cross-compile for Linux without having to copy the project to Linux. The steps to cross-compile can be found here: A new, community-hosted Unreal Engine Wiki - Announcements - Epic Developer Community Forums
Cheers