@glucose is correct, the hierarchy structure of the headings is set in the Style dialog.
To check or change this, open the Style dialog with the icon.
Check that the “Select” field shows the Style you are using for your Preface. If your cursor was in that line when you opened the dialog, it should already be selected.
Click on the “Enumeration / Table of Contents” tab and check the field “Hierarchy level”. In this example, the Style is set to “none” and so will appear at the top level.
However, if I check the same setting for the Style I am using for “Chapter 1”, you can see this is set to “Level 1”:
This combination can be used where, for example, you are writing a multi-part book and wish the top heading to be “Part 1” with the relevant chapters appearing underneath, then “Part 2” and associated chapters, etc.
If you are using automatically numbered chapters, you will need to change the hierarchy level of the Preface Style and set this to same as the chapter Style. Automatic numbering is set in the “Format description” field:
If you are not using automatically numbered chapters, you can change the Chapter Style to “none” or the Preface Style to the same as the chapter Style.
Once you have changed the Style setting and clicked “Apply”, your Navigator and Organizer should look like this:
How your heading Styles appear in the table of contents is also set is this dialog and on this tab. Make sure you have the Chapter / Preface Style mapped to the same “TOC” Style if you want these to also appear at the same level.