Storage Options tablets are not licensed for/by Google Playstore because they have not been fully tested by Google.
Consequently Google don't know what your tablet is or what it is compatible with so a lot of apps apart from some basic stuff all show as "not compatible with your device". Stock Excels usually show on the Google account as "Unknown Excel".
The main reason many Google Playstore apps show as incompatible with your Excel is that if an app misbehaves or crashes your tablet Google do not want you chasing them to complain about it. Therefore, when Google don't know if an app will work because they have not tested and approved your device, they choose to play safe and tell you it is incompatible. Chances are the app may well work OK without any problems if you can download it any other way and then sideload it to your tablet.
The only other alternatives are to use the 3rd party app providers suggested by Storage Options:
http://tinyurl.com/9mbt9ubor get involved with changing the fingerprint of your Excel to something else Google approved which is what I did, but this means getting under the bonnet and having a tinker and it is not without risk.