If you have 'Quotation Conditions' set to "Show Add to Quote only when price is '0.00'" and the product price is 0,00, it disables the 'Add to Cart' button automatically.
General - Auto Hide Order Button
If you have 'Quotation Conditions' set to "Show Add to Quote only when price is '0.00'" and the product price is 0,00, it disables the 'Add to Cart' button automatically.