1. Make sure you have an active internet connection (mobile data or wifi);
2. Tap on 'Courses' and then on 'Synced' tab and see if the course you are trying to download already appears there. If yes, delete the course:
IOS: Tap the 'More' icon (3 dots at bottom right of the cell) and select 'Delete Course'.
Android: Press the course name for a few seconds and a 'Delete Course' message will pop up, just tap 'Ok' to delete the course.
3 - Search for the course and try downloading it again.