Here is a screen shot of the privilege set where I have the field "event" set as modifiable. Not sure why its not letting me modify something that I gave permission to modify.
Thanks for the help
If security settings were preventing you from modifiying the field, the error message would read: "Your access privileges do not permit this".
The error message you are showing is due to some action on your part that is attempting to either directly or indirectly modify a field that is not modifiable.
A non modifiable field is either a calculation field or one that is set with the "prohibit modification during data entry" setting on the auto-enter tab in Field Options.
The tricky part may be in identifying the precise reason why your action is tripping that error. One example where the error could be tripped indirectly is that if you enter data in the bottom "add" row of your portal, this error message would pop up, if the foreign key field for the portal's table is not a modifiable field. This happens because that data entry causes FileMaker to try to create a new record in the portal's table and then link it to the current record in the layout by copying the primary key from the parent record into the foreign key field of the portal table's record. This last bit throws the error you see if the foreign key field is not modifiable.