Hi Dawid,
Welcome to Apple Support Communities! We appreciate you reaching out for help with this issue you're experiencing when attempting to upgrade your iCloud storage. Let's get to the bottom of it together.
To start, please tell us more about what's going on. Which device are you using when this occurs? Have you had an opportunity to restart the device, and join a different Wi-Fi network or cellular data, to see if the issue persists?
Will it be possible to attempt to upgrade your iCloud storage from another device?
We'd like to share the following guide with some helpful tips to help upgrade your storage:
Upgrade your iCloud storage plan - Apple Support
We're here for you if you've got any questions!
Have a great day.