وَأَنَّا كُنَّا نَقْعُدُ مِنْهَا مَقَـٰعِدَ لِلسَّمْعِ ۖ فَمَن يَسْتَمِعِ ٱلْـَٔانَ يَجِدْ لَهُۥ شِهَابًا رَّصَدًا
Asad Quran Translation
notwithstanding that we were established in positions [which we had thought well-suited] to listening to [whatever secrets might be in] it:8
and anyone who now [or ever] tries to listen will [likewise] find a flame lying in wait for him!9
I.e., "we failed notwithstanding our status as descendants of Abraham, and despite all our ability and learning".
As the sequence shows (and as has been pointed out in note  on 15:18 , this relates to all attempts at predicting the future by means of astrology or esoteric calculations, or at influencing the course of future events by means of "occult sciences".
Malik Quran Translation
Before this we used to find a seat in heaven for eavesdropping, but now eavesdroppers find shooting stars lying in ambush for them.
Yusuf Ali Quran Translation
`We used indeed to sit there in (hidden) stations to (steal) a hearing; but any who listens now will find a flaming fire watching him in ambush. 5734
What is the force of "now"? It refers to the early Makkan period of Revelation. It means that whatever excuse there may have been before, for people to try to seek out the hidden truths of the Unseen World through jinns, there was none now, as the perspicuous Qur-an had restored the Message of Unity and cleared religion of all the cobwebs, mysteries, and falsehoods with which priestcraft and pious fraud had overlaid it. The result is that such seekers after false hidden knowledge will find themselves confronted now by the flaming fire, which, like the shafts of meteoric light (see note), will lie in wait for and nip such priestcraft and black magic in the bud.
Mustafa Khattab Quran Translation
We used to take up positions there for eavesdropping, but whoever dares eavesdrop now will find a flare lying in wait for them.
Piktal Quran Translation
And we used to sit on places (high) therein to listen. But he who listened now findeth a flame in wait for him;
Waanna kunna naqAAudu minha maqaAAida lilssamAAi faman yastamiAAi alana yajid lahu shihaban rasadan