My Droplets started showing this error after updating to 1.4.5:
I’m on macOS 10.14. I have two saved Droplets, and both exhibit the same behavior, but they do actually run and produce the expected results, it’s just that I have to click through the error message each time.
Here’s one of the Droplets: Grayscale & JPEG.zip - Droplr
Further, if I click Unlock, I get this error:
Is the droplet on a disk with permissions or write access that you can’t change?
What happens if you copy the droplet to another location on your disk (like you’re home folder) and try and run it from there?
Strangely fixed by dragging the droplets from ~/Applications/Droplets to Downloads and back again 
Oh! This is a long standing bug with the Finder and the quarantine flag it puts on downloaded files.
The Finder is supposed to remove a little flag on apps (stored in the “extended attributes”) which tells it to treat this app a little more carefully, and let it do a little bit less, until it’s removed out of the Downloads folder. And when you move it somewhere else the Finder sometimes fails to do it’s job of removing the quarantine flag. So a quick move to another location and back can fix it.
Sorry you ran into this - but I’m glad you got it figured out. If someone else runs into it at least I’ve got one more thing for them to try for a fix.