كَلاَّ

On each meaning, on each of the two groups, those who desire this world and those who desire the Hereafter, We bestow what they want

مِنْ عَطَآءِ رَبِّكَ

A self-opinionated deed and an act of religious innovation, no matter how appealing, is not acceptable

In this verse (19), by adding the word: سَعْيَهَا (as due), it has been made explicit that every deed and every effort is not necessarily beneficial or acceptable with Allah. Instead, the only deed or effort credible is that which is appropriate to the essential objective (of the Hereafter). And whether or not it is appropriate can only be found out from the statement of Allah Ta’ ala and His Messenger ﷺ . Therefore, even well meaning efforts made by insisting on bland personal opinion and self-invented ways - which includes common customs of Bid'ah (innovations in established religion) - no matter how good and beneficial they may appear at their face, but are not appropriate to the effort made for the Hereafter. Consequently, they are neither acceptable with Allah nor are they of any use in the Hereafter.