You see the non compatability/not optimised sign on Android when your device is using an older version of Android and not the version that the developer wants you to use. Although it says that, sometimes it will let you download it anyway, if so give it a try, the chances are it will work fine. I have the app on Android 4.2 and Android version 6.0. And both working fine.
Alternatively you can upgrade your Android version to a later version on your tablet.Go to settings,about device, software update. Back up any important data, or move or save on your SD card in case of a problem.
Alternatively you can use the PC version
This link hopefully will take you to the web version of the app which you can use on your PC or alternatively on Chrome or Firefox browser on your Android device.
https://www.iberdroladistribucionelectr ... html#loginIts a hard slog but keep going its worth it.
When you do get to register it asks for a Cups No. Its on your bill as Unified supply point Code. Its a long number at the bottom of Contract Data.Just mentioned this as its not easy to spot.
I had numerous goes at registering as they had my NIE as my Passport number and Visa Versa, so had to keep permutating until I got to the right combination.
Some forums will not allow www. links in posts so if link does not work let me know and can email it to you.
Let me know how you get on.
Mike