The host probably just wasn’t detected as alive. Please check the following resources below.
I’m also wondering why this topic was put into the “Open Scanner Protocol (OSP)” category and with so many wrong tags (it is clearly not a OSP related topic and also not a duplicate). Maybe you could edit the topic, remove any unnecessary tags and put it into the better fitting Scanning / scan configuration - Greenbone Community Forum category?
https://docs.greenbone.net/GSM-Manual/gos-20.08/en/scanning.html#hosts-not-found
https://docs.greenbone.net/GSM-Manual/gos-20.08/en/scanning.html#alive-test