You posted the question to the CLOSED Reader/Acrobat DC for mobile (Read Only) forum. But Adobe Reader or Acrobat DC mobile products (for iOS, Android) are not an eBook reader.
Adobe Digital Editions for iPad is available from the App Store.
I will move this to the Adobe Digital Editions forum so that the experts in the forum can assist you.
This error comes when you previously downloaded the book with some authorization credentials in ADE.
Now again if you try to download the book from same download link but current ADE authorization is different, you will receive this error.
is this the case ??
By any chance you downloaded the book somewhere else also?
Make sure ADE is authorized with same credentials, as it was authorized when book was downloaded for the first time.
Then this error will not come.