What is the exact wording you see? Might it perhaps be complaining of not enough iCloud storage rather than iPad storage?
You can check your available iCloud storage from iPad settings:
Settings > [Your Name / AppleID] > iCloud > Manage Account Storage
Unless you have subscribed to a higher “paid” storage tier, you will be limited to 5GB of iCloud storage. While this may be sufficient for basic device synchronisation, if you are storing iCloud backups, photos or files in iCloud, the “free of charge” storage quota is very easy to fill.
These support pages should prove to be helpful:
About storage on your device and in iCloud – Apple Support
What's the difference between device storage and iCloud storage? – Apple Support
Manage your iCloud storage – Apple Support
How to check the storage on your iPhone, iPad and iPod touch – Apple Support
Clear Other storage on your iPhone or iPad - Apple Support