Starting nafl seated is permissible despite ability to stand, though standing is superior.
Sitting after initially standing is allowed by Abu Hanifa but, absent an excuse, not allowed by Abu Yusuf and Muhammad.
The standing requirement for fard prayer is distinct and not the subject of this disagreement.
Opening interactive controls...
Voluntary Prayers: Regular Sunnahs, Nawafil and Juristic Differences
Starting nafl seated is permissible despite ability to stand, though standing is superior.
Sitting after initially standing is allowed by Abu Hanifa but, absent an excuse, not allowed by Abu Yusuf and Muhammad.
The standing requirement for fard prayer is distinct and not the subject of this disagreement.