When you are deploying an Operating System using SCCM task sequence you may notice that the options under Make available to the following is greyed out and by default the value is set to Only Configuration Manager clients.

You will see this behaviour because the newly created task sequence doesn’t have an associated boot image.

To fix this issue, go to the task sequence which you are trying to deploy then right click then open the properties. Under the advanced tab, tick Use a boot image then browse to the required boot image. Now, all available for options for OSD task sequence deployment options will be visible.