﴿ جَزَآءًۢ بِمَا كَانُوا۟ يَعْمَلُونَ ﴾

“[And this will be] a reward for what they did [in life].”

a reward jazā’an is an object denoting reason or a verbal noun with the operator being an implicit verb in other words ‘We have appointed for them the mentioned as a reward’ or ‘We have rewarded them this’ for what they used to do.