You'll only be able to download the By Miles app if you're based in the UK. If you're in the UK and still can't download, this is most likely because you don't have a UK payment profile in Google Play.
This might happen if you don't have a UK sim card, you purchased your device in another country, or are using non-UK payment details.
Here's what to do on an Android phone:
Open up the Play Store app
Click the menu icon (top left)
Go to Account
Click Payment methods
Click More payment settings
On payments.google.com, click the menu icon
Country/Region should display as United Kingdom (GB). If it doesn't, click the pen icon and follow the steps to create a new payment profile. Once this is done, make a paid purchase on Google Play.
You should now be able to download the By Miles app.