﴿ أُو۟لَٰٓئِكَ أَصْحَٰبُ ٱلْجَنَّةِ خَٰلِدِينَ فِيهَا جَزَآءًۢ بِمَا كَانُوا۟ يَعْمَلُونَ ﴾

“it is they who are destined for paradise, therein to abide as a reward for all that they have done.”

Those will be the inhabitants of Paradise abiding therein khālidīna fīhā is a circumstantial qualifier as a reward jazā’an is in the accusative as a verbal noun because of the implied verbal action that is to say yujzawna ‘they will be rewarded’ for what they used to do.