However the checkbox next to "Leave marker file in place of remediated file" is shaded out.
Make sure you have both Endpoint Discover and Endpoint Prevent license via System / Settings.
Note - there appears to be a defect in some 11.5 licenses, for which the presence of the Endpoint Discover & Prevent license files are not enough.
If this is the case, the addition of the Network Protect license file will allow marker file configuration. For more info, please see eTrack 2782223.