Can the "Tax Ref. Number" field on the Tax Profile screen be made a mandatory field?
Yes. To make the Tax Reference Number field a mandatory field on the Tax Profile screen, enable the "Make employee tax reference number mandatory on the employee tax status screen." checkbox under the Employee required field options section on the General Settings screen.
Navigate > Configuration > Basic Settings > Company Settings > General Settings > Employee required field options
Navigate > Employee > Basic Information > Tax Profile > Tax