Sign out the APPLE ID in "Settings," the pop-up message will disappear.
If the issue still exists, please shut down your device and restart it.
However, it Solution 1 does not work in your case, please try solution 2.
1. Delete the app. (NOTE: Please do not forget to back up your files! You can back up files by using iTunes or iFile.)
2. Sign out the APPLE ID in "Settings."
3. Restart your device.
4. Reinstall the app.
5. Make sure that the APPLE ID is consistent in both "Settings" and in "App Store."
Before deleting apps from your device, if you wish to back up the files and documents, may we suggest you sync your device with iTunes to drag out and save your files in the PC.
1.Connect your device to your PC/Mac, open iTunes or iFiles.
2.Go to the device page, choose "applications" and search the target app, e.g. PDF Connoisseur (shown in the picture below)
3.Tap on the app icon, you can see all the files and documents your saved in the app.
4.Back up your files.