You need to check field definition details in Manage | Database | Fields.
The typical case for this error message is when a script tries to modify the value of a field of type calculation. But field modification can also be prohibited as a field option inside manage database and if I recall accurately, it's the same message.
And what can be maddening with this message is that the error message does not name the field and this leaves you guessing at times as to which field is meant by "this" field.
I am running into this issue with a conditional dropdown, I've checked and re-checked all the spots that I've seen mentioned that could inhibit modification (Manage -> database -> fields, Edit layout -> allow entry of other values),a nd have it set up according to the guide provided for conditional lists. I don't know why I'm having so much trouble with this.
It's department, then name (Employee list):
Admin Damon Admin Matthew Sales Sean Sales Mike CS Carla CS Fernando Tech Craig Tech Wes Tech Gardenia
Department works just fine, can change when creating a new record, but when you select a different employee it pops back to the old one and then displays that error message... I know it's something really dumb, I just can't see it.
Any help would be most appreciated.
Thank you as always!
I figured it out. Sorry for making you read that! I had the Value list pointed wrong.