RawTherapee uses one processing profile by default for raw photos, and another one for non-raw photos.
RawTherapee 5.4 uses Auto-Matched Curve - ISO Low.pp3 for raw photos.
Versions of RawTherapee prior 5.4 used Default.pp3 which no longer exist.
The message is informing you that the default profile for raw photos, which you most likely still have set to the old Default.pp3, no longer exists.
Some other default settings have also changed between 5.4 and earlier versions. The easiest fix to this, and the one I would recommend as it will also fix other possible issues which you haven’t detected yet related to new and better defaults, is to delete your current “options” file: http://rawpedia.rawtherapee.com/File_Paths#Config