Project is Missing Plug-in files message will occur if a plugin(s) has been deactivated prior to loading the project:
To resolve this:
1. Ensure your MPC is connected to Wi-Fi/Ethernet
2. Log in to your inMusic Profile via Menu > Preferences > Activations
3. Then activate the 'missing' plugin(s)
4. Once done, go back to the plugin track and select the 'missing' plugin(s):
5. The 'missing' plugin(s) will appear again as normal in your project: