Thanks for asking the Apple Support Communities for help with your storage issue. I am happy to offer two articles to help you:
1. How to check the storage on your iPhone, iPad, and iPod touch
2. Manage your iCloud storage
If neither article solves the issue, please provide some more details as to what you see, where you see it, and/or what steps you are doing to see this warning icon. You can provide a screenshot as well, but please ensure your personal data is protected and not included in the screenshot. How to take a screenshot on your iPhone, iPad, and iPod touch
Thanks and have a wonderful day.