Nowadays, this authority is not delegated to the current generation's Sages, and thus the Torah can not be commentated on, in matters concerning the Halakha, if it is in contradiction to Chazal's commentary. Earlier on, up until the midst of the Tannaim era, when there was a Sanhedrin (a Jewish law court), Chazal had also the authority to decree predestinations and to enact new religious regulations, in any matter they saw fit, concerning issues that were not included in the written "Torah", or were not handed at Biblical Mount Sinai.
This authority is no longer delegated to the current generation's Sages, so the Torah cannot be commented on in matters of Halakha if it conflicts with Chazal's commentary. Earlier, until the Tannaim era, when a Sanhedrin existed, Chazal also had the power to make decisions and create new regulations as they deemed fit concerning issues not included in the written Torah or not revealed at Mount Sinai.