The specific error you report will appear if your script attempts to modify a field that is either specified in field options as not modifiable or which is defined as a calculation field.
If you can't spot why this is happening, you may need to debug your script step by step to see where/when the error message is popping up. If you have FileMaker advanced, enable the script debugger, then run the script and step through it one step at a time until the error message appears.
If you do not have advanced, add some show custom dialog steps that pop up unique messages so that you can see which ones pop up before you get the error message. You may have to do this more than once to narrow this down to a specific script step if you have trouble spotting the exact script step that's the culprit.
Remember that it may not be any of the 3 fields in your custom dialogue. It is possible (even probable, as you've checked those 3 fields) that the custom dialogue is perfectly correct, and it is what you are doing with data (and maybe not even that data) further down the script that is causing the problem. (Hence Phil's suggestion is more comprehensive that simply suggesting you re-scrutinise those 3 fields.)
It was the "Date Shift ends field" it was a calculation field that could be modified in the custom diolog window. Thanks guys! :)