Unfortunately there are any number of reasons that you might be getting this error. One that leaps to mind is that the offline app requires you to run it online at least once before it becomes available offline. Try loading it with a stable internet connection the first time around, that might help.
There are also a number of configuration issues that could cause this. These range from enabling the offline profile in your project to correctly configuring your app package as described in the documentation ( https://world.mendix.com/display/refguide6/Configuring+Hybrid+Mobile+Apps+To+Run+Offline ).
Hope one of the above fixes help. If not, please expand on the exact steps you took to get to this point and which issues you have already ruled out.