In certain reports you can not add column such as balance sheet,profit and loss etc.it is also depend on Model that your are using it should be Standard or Enterprise as in Basic Model Custom column option is not available only columnar report option is avaialable for some reports and these are Accounts Register,inventory Register,Cash or Bank Book,Stock status,Party Item Analysis and Parameter Stock Status.Make sure if you add custom column in report then it should be login from Super user of Your company.