Erecting and spreading (an-naṣb wa-l-farsh) in prayer

Answer (and Allah grants success):
Supererogatory prayers are built upon ease and facilitation; hence they are valid on a mount, valid sitting without excuse, and for the traveler without facing the qiblah.
What I think they intend by “obligatory” here is not “what earns reward if done and incurs punishment if left,” but rather that it is required for validity: if omitted, the prayer is invalid—but if a nafl prayer is invalidated, making it up is not required.
There is another view: once a morally responsible person enters a supererogatory prayer, it becomes obligatory for him to proceed in it, per His saying: “And do not invalidate your deeds.” [Muḥammad:33]. Accordingly, he must perform erecting and spreading and anything else without which the prayer is not valid. Thus the nafl becomes obligatory by entering it, similar to the supererogatory ḥajj.
Source : Min Thimār al-ʿIlm wa al-Ḥikmah vol.1