I found a working solution. Liberty 23.x does not expose TransactionManager over JNDI, you need to use reflection to obtain TransactionManager.

This bean definition helped me configure JtaTransactionManager.

    @Bean
    public JtaTransactionManager transactionManager() throws Exception {
        InitialContext context = new InitialContext();

        UserTransaction utx = (UserTransaction) context.lookup("java:comp/UserTransaction");

        TransactionManager ibmTm = (TransactionManager) Class
            .forName("com.ibm.tx.jta.TransactionManagerFactory")
            .getDeclaredMethod("getTransactionManager")
            .invoke(null);

        return new JtaTransactionManager(utx, ibmTm);
    }

I found the answer here: https://stackoverflow.com/a/79363034/30908754