why cant i download ios 15 even though i have enough space?

You are not going to be able to download and install the iOS with only 2GB remaining on your iPhone. You will need approximately 6GB+ to install the update. You can even try and update manually using your computer. Learn how to update your device manually ➔ if you're using a Mac with macOS Catalina or later, or ➔ if you're using a Mac with macOS Mojave or earlier or a Windows PC.

Axel F.

You need way more than 2GB of free space to download iOS 15. The update is more than 3GB. You need at least 5 - 6 GB of free space to update over the air.

You need to either delete a lot off your phone as having only 2GB of free space is not much free space to begin with or try to update your phone using your home computer with iTunes.

You should give serious consideration to thinking about your next phone and making sure it has much more memory than your current phone.