Correct. Fields of type calculation cannot be modified. You can't click in the field and edit the data and a script can't use an "insert" type step nor can you use the better option of using set field to modify the field.
You might be able to change your calculation field into a number field with an auto-entered calculation that then permits direct edits, but I'd just change the calculation field's calculation and get rid of the script and script trigger as it is unecessary.
Make your new calculation something like this:
Case ( MyLayout::DayType = "Holiday" ; 8 ;
Mylayout::DayType = "sick day" ; //put value or calculation for sick day here
Mylayout::DayType = "vacation" ; //put value or calculation for vacation here
Mylayout::DayType = "regular" ; //put the calculation that you already have in the field here )
You may also want to set up an auto-enter setting that enters "regular" into DayType so that it's the default value in each newly created record.
Works perfectly! and you just answered my next question about "regular" Thanks again :)