can't install iOS 15 update on iPad

If your internal storage is full, your only option is to better manage the storage that you have - and delete unused Apps or unneeded data. 

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

“Offloading” Apps can be a helpful method of creating temporary space for the software update.

https://9to5mac.com/2020/01/21/how-to-offload-apps-in-ios-to-save-space-without-deleting-their-data/

Additional information about creating sufficient space for installation of a software update can be found here:

If you need more space for an update on your iPhone, iPad, or iPod touch - Apple Support

Updating your iPad using a PC or Mac computer can reduce the amount of free-space necessary to perform the update - as the initial download, unpacking and verification is performed on the computer prior to installation. However, even this method is not possible if your iPad has too little free space.

On the iPad delete apps or other content to make room.

On iTunes on the computer, select set up as new, and then you’ll get the option to update. It will not affect any data on the iPad by selecting setup as new.