وَكَمْ مِنْ قَائِمٍ لَيْسَ لَهُ مِنْ قِيَامِهِ إِلَّا السَّهَرُ distinguishes the outward action of standing at night from its intended spiritual effect. An act of prayer may be performed while sincerity, humility, and reform receive too little attention.
قَائِمٍ (one standing) and قِيَامِهِ (his standing) are related to the root ق و م; the first is an active participle and the second a verbal noun.
Allowing prayer to strengthen honesty, humility, and the fulfillment of duties is a practical response to this warning.