Google-sheets – Enabling script to run as author in a shared Google spreadsheet

If you add the following formula in cell D2:

=ARRAYFORMULA(FILTER(C2:C;C2:C<>"")*1.2)

then it will take on the complete column, via the array. The FILTER function, is to make sure no empty cells are taken into account.

I've added the result into the file you shared with us.

This is one of restrictions imposed on custom functions: they cannot perform tasks that require authorization, such as accessing user's files. From the documentation:

Unlike most other types of Apps Scripts, custom functions never ask users to authorize access to personal data. Consequently, they can only call services that do not have access to personal data, specifically the following:

[a list that does not include SpreadsheetApp]

To use a service other than those listed above, create a custom menu that runs an Apps Script function instead of writing a custom function. A function that is triggered from a menu will ask the user for authorization if necessary and can consequently use all Apps Script services.

Another solution, perhaps more suitable for your case, is to use ImportRange in your spreadsheet with the function, and access those cells. A custom function can access read-only methods of Spreadsheet service, such as getSheetByName, getRange, etc.