A property (variable) is defined for a papyrus script, but its definition does not exist in the script.
Missing properties are also reported in the papyrus logs as "X does not have a property named Y, property skipped."
This can happen in case scripts are updated or being overwritten.
It can be a sign of load order conflicts of two mods requiring different versions of a script or of a mod author not properly error checking and cleaning up after making changes.
In case the script property does not exist in the script anymore intentionally, remove the no longer needed entry from the script properties on the mentioned record with xEdit.
In case the error occurs with paid mods from CreationClub, consider using the Unofficial Skyrim Creation Club Content Patches.