When opening the latest KaarbonTech SMART apps, you will see this popup: When it appears, tap Open System Settings..:

Then, on the screen that appears, scroll down until you see your KaarbonTech SMART app and turn the switch on the right hand side to on:

Then you can tap the back arrow to return to the app. 

If you simply return to the app without enabling this permission, you can enable it manually through the main tablet settings screen:

Settings > Apps > Three Dots > Special Access > All Files access

This gets you to the screen above.