sCO2Flex (H2020 grant agreement #764690) is a project that addresses the necessities to upgrade existing fossil fuel plants to integrate renewable energy sources increasing the overall efficiency in power generation plus avoiding the use of water. Deliverable of the project is the development and validation of all main components of a sCO2 Brayton cycle capable to provide 25MWe at 100% load ensuring a wide plant flexibility (from 100% to 20% of the electrical load). Moreover, it includes testing of a centrifugal compressor prototype. The present work will describe several new challenges faced by OEM during the design of turbomachinery for this application. The first is linked to the operating conditions of the main compressor, which achieves the highest cycle efficiency in a region close to the CO2 critical point. In these conditions, possible co-existence of different phases may occur with sudden change of speed of sound causing uncertainties in performance predictability. To minimize these uncertainties and at the same time fulfil loop flexibility needs, specific design for compressor internals was adopted and will be discussed in this article. The present work will conclude with highlights on the full size 5 MW prototype compressor that was manufactured and tested in OEM test facility in Q2 2021. The test replicated exactly the nominal operating conditions, specifically vicinity of the compressor suction to CO2 critical point. This has enabled a direct validation of compressor performance predictability and at the same time allowed to verify the compressor rotordynamic, mechanics and aerodynamic behavior.