Quick summary

Your Lightroom catalog is locked by Lightroom or another process.

Error message

Your catalog is locked. See this Adobe guide. Close Lightroom, and try to download again. If Lightroom is closed, open it, and then open your catalog. Close Lightroom, and try to download again.

How to fix this

1. Try to unlock your catalog according to this Adobe guide. 
2. If Lightroom is open, close Lightroom, and try to download again.
3. If Lightroom is closed, open Lightroom and then open your catalog. Close Lightroom, and try to download again.