Hi Khopi,
Welcome to the community!
This is about free diskspace on the server hosting SonarQube. And while 19.8gb would seem to be plenty of free filespace, unfortunately, this works on percentages. So you’ll need to free up a little more filespace - I think the 10% free mark is what you need to hit. And then restart SonarQube. The indices will automatically be unlocked when you do.
Ann