Assuming that you have enabled the RTCOUT pin in the pin configuration, add this line of code to your project.

R_RTC->RCR2_b.RTCOE = 1;

For example:

void hal_entry(void)
{
/* TODO: add your own code here */
fsp_err_t err;

err = R_RTC_Open(&g_rtc0_ctrl, &g_rtc0_cfg);
if( FSP_SUCCESS != err )
{
__BKPT(1);
}

err = R_RTC_CalendarTimeSet(&g_rtc0_ctrl, &sys_time);
if( FSP_SUCCESS != err )
{
__BKPT(1);
}

R_RTC->RCR2_b.RTCOE = 1;

while(1);

}

Would have thought that an API may exist for this, or would be enabled by default when you enable the pin in the pin config.