يَٰٓأَهْلَ ٱلْكِتَٰبِ قَدْ جَآءَكُمْ رَسُولُنَا يُبَيِّنُ لَكُمْ عَلَىٰ فَتْرَةٍ مِّنَ ٱلرُّسُلِ أَن تَقُولُوا۟ مَا جَآءَنَا مِنۢ بَشِيرٍ وَلَا نَذِيرٍ ۖ فَقَدْ جَآءَكُم بَشِيرٌ وَنَذِيرٌ
Yā ahla'l-kitābi qad jāʾakum rasūlunā yubayyinu lakum ʿalā fatratin mina'r-rusuli an taqūlū mā jāʾanā min bashīrin wa lā nadhīr · Fa-qad jāʾakum bashīrun wa nadhīr · Wa'llāhu ʿalā kulli shayʾin qadīr
"O People of the Book! In a period when the succession of messengers had lapsed, Our messenger has come to you, making things plain, so that you may not say, 'no bearer of good news and no warner came to us'. Now a bearer of good news and a warner has come to you. God has power over all things."
The sense of the root: to slacken, to weaken, to lose speed. Futūr — slackness (the Turkish "fütur getirmek", to lose heart, is from this root); fātir — slackened, gone still.
And the picture the word gives matters: fatra is not "a void in which nothing happens" but the interval in which the motion slows. The word conveys not a clean break but a slackening.
عَلَىٰ فَتْرَةٍ مِّنَ ٱلرُّسُل — "upon an interval in the messengers". That is, the messenger comes upon this interval.
That is, as the ground for the sending of the messenger, a sentence the addressee might say in future is put forward. The proof is built as the closing off, in advance, of an objection yet to come.
This is a structure repeated in many places in the Qurʾān: responsibility is not established without knowledge having been given. Among the relevant verses one may count al-Nisāʾ 4/165 and al-Isrāʾ 17/15 (wa mā kunnā muʿadhdhibīna ḥattā nabʿatha rasūlā); it was treated in al-Isrāʾ, and I rely on that treatment.
بَشِير and نَذِير — two descriptions. The root ب-ش-ر was treated in the preceding verse. The root ن-ذ-ر: to give warning of a danger beforehand. It was treated in al-Muddaththir and at al-Najm 53/56.
A point of word order: at the head of the verse the two words come negated and indefinite (mā jāʾanā min bashīrin wa lā nadhīr — no bearer of good news and no warner at all); at its end they come affirmed and again indefinite (fa-qad jāʾakum bashīrun wa nadhīr). The same two words, in the same verse, first as the claim and then as the answer. The answer is given in the objection's own words.