The checkbox is a calculated field exists only in the data file
If you mean by the above that the field with the checkboxes is a Calculation field, reflecting the contents of a Text field in the related file, then that is why you can't modify it---a user can't modify a calculation field. Perhaps what you need is to create a Value List using the values in the Text field in the related file, then make the checkboxes field a Text field showing the checkboxes from the Value List.
Thank you, keywords for the reply.
Is there a tool to search the entire database (script, field, layout,... ) for this specific field? I inherit this from someone else and not very clear on its purpose.
Yes. The tool is Filemaker Pro. Switch to Layout mode and double click the field. The field name will be displayed in a list with the name of the table above the list.
Now look in the File menu File->Manage->Define Database. Open the relationship graph and locate the table. Hover the mouse over the title area of that table to see which file it comes from and what name it has in that file. Close the window by pressing the cancel button.
Now locate the other file. It will appear in the Windows menu. Go to File menu File->Manage->Define Database again. This time select the Tables tab and double click on the table name. That will display the fields tab. Locate the field you are interested in.
If you have Advanced you can create a database design report and search that. Choose the HTML version.