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This paper proposes a simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) enabled scheme for a cell-free symbiotic radio (SR) system. In the considered system, multiple access points (APs) are positioned on opposite sides of the STAR-RIS for cooperative primary transmission to the information receiver (IR). The STAR-RIS is set up to support the primary transmission by providing extra transmission links through simultaneous transmission and reflection. Additionally, it can send its own data to the IR by catching a ride over the primary signals. We take both perfect channel state information (CSI) and imperfect CSI scenarios into consideration. For each scenario, we formulate a primary system energy efficiency (EE) maximization problem to optimize each active beamforming constructed by the APs and the passive transmission and reflection beamforming established by the STAR …