Hello Accraaccademy,
It sounds like the answer to your question is in your description. If you're seeing a message "you do not have enough storage in your device" when you try to download an update, then that is why you can't download it.
If you've used up all the storage on your device, you'll need to start deleting things on it to free up some space. You can get an overview of what is using your storage in Settings > General > iPhone Storage.
How to check the storage on your iPhone and iPad
You can see how much space the update requires when you go to Settings > General > Software Update. You'll need to make sure you have at least that amount of storage available. Keep in mind you should always have at least 1 GB of free space on the device in order for it to function properly.
Cheers!