Oracle Database 23ai includes several new features for Oracle Real Application Clusters (RAC) that are not supported in Standard Edition 2 (SE2):

Oracle Standard Edition 2 (SE2) no longer supports RAC starting with Oracle version 19c. This is due to a technical change in the underlying software. 

Oracle has outlined several options for clients currently using a RAC configuration with their SE2 deployment, including staying on 18C or converting the RAC Database into a Non-RAC Database instance.