This has already been reported by a user with a different name. See here for details: Script Step Failure when developer has 'Manage Database - Specify Calculation' Dialog box open
New Record is not the only step that will fail when this action locks the entire table.
Thank you for your posting.
The link provided by PhilModJunk (Thank you!) has been updated. In essence, FileMaker Pro will not allow users to create records while another user is potentially modifying the table schema. FileMaker Pro will return the correct alert and error code unless Set Error Capture [On] is enabled.