The myatmo Premium version and the data export function require that iCloud Drive is activated for myatmo. This is done in the following way:

1. Open the iOS settings
2. Tap onto your name or 'Apple ID, iCloud, iTunes & App Store'
3. Tap on iCloud
4. Enable 'iCloud Drive'
5. Scroll down and activate 'myatmo'

 
After that, myatmo Premium can be activated and all data export functions in myatmo can be used. If you do not want to use the data export feature after activating myatmo Premium, you may disable iCloud Drive again.

For more information on how to activate iCloud Drive on your device, please check this Apple Support page.