As a general rule, it's good to "assume' that you need 2-3 times as much space available on your iPad when downloading an app. So, if you are downloading an app that is 1GB in size, you need 2GB space available (sometimes even close to 3GB). It's the design of the download process as the app/iPad need the space to do the download.
All I can recommend is you take a look at the size of the app you are trying to download (info at the bottom of the app description in the AppStore). If you don't have 3x the space on your iPad, you will need to remove apps/files until that space is available. Then, you can try the download again.
Marilyn