Answer:
This message means that the linked project folder has not been found (see Image 01). For example, this may happen if the folder has been renamed or relocated after creating the project. If the directory path is no longer valid, the project folder will be displayed as transparent.
If you do not need this project anymore, you can use the [Disconnect] button in the dialog box to remove it. However, if you continue to use the project, you should close the dialog box by clicking the [Skip] or [Skip All] button (if several folders are defective).
You can use the "Connect Folder" command in the Project Manager menu "Project" to integrate the folder as a project in Project Manager. Use the [Browse] button to set the file folder and the project name; see Image 02.
In the Project Manager menu, you can also create a new project and connect it to the new path.