Missing third party updater
This is the message that comes up every time you use an application that has a so-called "updater" in the project. If you do not have the application installed, Revit will notify you when you open the project (as there may be elements in the project if not updated).
This is not an error message! An information or warning. If you don't want to see this one more time in the project, you choose "Don't warn about this updater again ..." That option does exactly what it says.
In the example above, it seems to be the "updater" for REX that it is being warned about. Someone has used REX in the project and now you open the project without having REX installed.
In other Naviate applications we make our "updaters" silent, that is, they configure them so that a warning should not come. Autodesk has not done this with the REX application.