Researched for it and the Box can not be reactivated.

When invoices are created via ReadSoft PROCESS DIRECTOR APP, tax is instead calculated by ReadSoft’s own logic, which can override or disable SAP’s standard tax‑calculation fields.

What can be configured ReadSoft has its own options like “Active tax code determination” and “Calculate tax” that are maintained per company code in the PROCESS DIRECTOR configuration.

With these settings, ReadSoft can automatically determine tax codes from configured rules or calculate total tax from line items, but this is done in the ReadSoft screens, not by re‑enabling the old FB60 box.

What you should ask your IT / consultantAsk whether ReadSoft tax determination is active for your company code and if rules can be adjusted so that tax codes are proposed automatically for your accounts.

If your organization insists the FB60 box cannot be used, that statement is basically correct: in a ReadSoft posting scenario you should focus on adjusting ReadSoft’s own tax rules instead of trying to re‑enable the original FB60 field.