If you are not able to turn in the full Marni Stones you need to disable auto-sort and move them to the first slot of your inventory. It worked for me.
Install the Black Desert Launcher if the game doesn't start.
The launcher will appear if it's installed.
If it doesn't, try to run your downloaded launcher.
1 Run BlackDesert_Installer_NAEU.exe to install the Black Desert launcher.
2 Start the game once installation is complete.
If you are not able to turn in the full Marni Stones you need to disable auto-sort and move them to the first slot of your inventory. It worked for me.