Why is the OVO app not available download for phone running Android 6?

I suspected as much. Just another example of corporate not supporting long term customers and trying to force them to upgrade.  As I do not wish to buy another phone and it will not upgrade past 6 I'll either have to put up with it or look for another supplier. Probably the latter 

Actually, OVO does try to support Android versions for as long as possible, but like all developers, they do have to eventually drop support for old versions. Your phone is almost ten years old and is on an Android release that has had no security updates for at least seven years now. If Google forces developers to drop support for an old Android release, the developer has no choice but to do so or not be able to update the app.

It’s also possible that some of the tools OVO uses to build the app no longer support Android 6, which again forces a version bump. Other suppliers are starting to remove support for Android 6 from their apps as well.

As Julia mentioned, the best alternative is to use the web version via https://my.ovoenergy.com but I should mention that as your phone gets older, this problem will get worse as more apps stop updating for you. In actual fact, your phone originally came with Android 4.4.4 KitKat when it first launched and had two years of major upgrades to Android 5 and 6, which is when Sony is known to drop support (their usual policy is two major version upgrades for flagship models, then a bit of security support). So that indicates your phone is even older than it might first seem.

I’m really sorry, but as a forum volunteer there’s not much else I can do. OVO won’t have changed the requirements without considering it first, but it was inevitably going to happen - it was just a case of when, not if.