If you are unable to pay for coins in the store, there are several possible causes.

In-app purchases are restricted

In-app purchases may be restricted on your device. Please check your App Store (iOS)/Google Play (Android) settings to see if you are able to purchase in the app.
For more information about the setup, please refer to this article.

(Android) Pikmin Bloom is not linked to Google Play services

Pikmin Bloom must be connected to Google Play services in the background in order to process purchases on Android.
On some devices, background data for Google Play services may be disabled in the data server settings.[Settings] > [Data] > [Google Play Developer Services] and make sure background data is enabled.
For more information about Google Play Developer Services, please visit Google's support page.

Data Connection Issues

Make sure your device is connected to a reliable network with stable communication. If it is unstable, try connecting to a different network and access the shop again.