Having looked at the specs of this phone I see the screen resolution is 800x480 pixels. This is VERY low, probably the lowest I have ever seen on a phone (my phone is 2560x1440 pixels).
My guess this is why the app wont work
But also sadly this phone was not a good choice.
It only has 8Gb of ROM memory, but while this may not be the cause of your problems it is far too little for a phone nowadays. Android and the built in apps probably takes over half of that before you start.
Also Android 5.1 is over 3 years old which is very old in smartphone terms.
Most modern apps will be tested on the latest version of Android but may not be tested on the older versions.