B737NG_Pilot
The RWY lateral mode provides the YAW BAR on PFD. A director-type indication to help track the localizer signal during take-off roll if necessary. For instance, when visual reference is compromised momentarily.
It first appears after TLs are set for take-off, assuming necessary conditions are met. If you were engaging FLEX whilst more than 20° offset, the HAL just thinks perhaps it's not a centre-line roll you are trying to perform and won't show the yaw-bar (RWY FMA). Can't blame him.