Make sure you don’t have an active restriction on buying in-app purchase products.
You can find out more information regarding in-app purchases here:
iOS: https://support.apple.com/en-gb/HT204396
Android: https://support.google.com/googleplay/answer/1050566?hl=en
Alternatively you can purchase a subscription online here:
https://payment.languagedrops.com/purchase
Language learners from China and Iran: We are hard at work improving the app and adding more payment options, so please stay tuned!