( وَأُمْلِي ) (And I will give them time): Allah (swt) does not take them to task right away, rather He delays and postpones.
(to plan something) is the act of planning or arranging something in a subtle manner.
(Firm) means powerful or strong.
Allah (swt) grants them respite, then His (swt) subtle scheme is exceedingly firm, and no one can overcome it.