The target is the field into which you are attemptint to enter data. If you are manually attempting to edit a field, go to layout mode and look at the field. I predict that you will see a "missing" error message in place of the fieldname.
If you are using Set Field in a script, you'll need to take a closer look at your script, what layout is "current" at the point when the script step executes and what relationships are in place linking other tables to the current layout's table occurrence.
And don't forget the possibility of a script trigger performing a script that produces this error message when you edit a field on your layout.
Bit in the buttissimo by a script trigger used improperly.
So much to learn, so little patience, LOL.
Thanks again Phil.