So be patient, [O Muhammad]. Indeed, the promise of Allah is truth. (40:55)

Almuntakhab Fi Tafsir Alquran Alkarim

Therefore, be patient O Muhammad and bear with them their faults and limitations; the promise of Allah, your Creator, is the truth personified. Invoke Allahs forgiveness for any instance where you went short of a standard or showed apprehension, and praise Allah, your Creator, in acknowledgement of His Divine favours and extol His glorious attributes evening and morning

Tafseer Tafheem-ul-Quran by Syed Abu-al-A'la Maududi

(40:55) Be steadfast, *71 then, (O Prophet), Allah's promise is true. *72 Seek forgiveness for your shortcomings, *73 and celebrate the praise of your Lord, evening and morning. *74

So be patient, [O Muhammad]. Indeed, the promise of Allah is truth. meaning

*71) "Be patient"; "Bear up with a cool mind against these hardships through which you are passing."
*72) The allusion is to the promise made above in the sentence: "We do help Our Messengers and the Believers in the life of this world."
*73) A study of the context shows that here "error" implies the state of impatience that was being caused to the Holy Prophet on account of the severe antagonism, especially the persecution of his followers. He earnestly desired that either some miracle should be shown, which should convince the disbelievers, or something else should soon come down from Allah, which should quell the storm of opposition. Though this desire in itself was not a sin, which should have called for repentance, the high rank with which Allah had blessed the Holy Prophet demanded great grit and will power; so the little impatience shown by him was regarded as inconsistent with his high rank. Therefore, he was required to ask Allah's forgiveness for showing the weakness and was told to stand his ground like a rock as a man of a high rank like him should.
*74) That is, `Glorification of Allah with His praises is the only means by which those who work for the sake of Allah can get the strength to resist and overcome the hardships faced in the way of Allah. ' Glorifying the Lord morning and evening can have two meanings: (1) `Remembering Allah constantly and perpetually:" and 12) `performing the Prayers at the specific times." And in this second case, the allusion is to the five times of the Prayers, which were enjoined on all the believers as a duty some time after the revelation of this Surah. For the word 'ashiyy in Arabic is used for the time from the declining of the sun till the early part of nigh, which covers the times of the four Prayers from Zuhr to 'Isha': and ibkar is the time from dawn to the rising of the sun, which is the time of the Fajar Prayer. (For further details, see AI-Baqarah: 3, 43, 45, 238; Hud: 114; AI-Hijr: 98-99; Introduction to Bani Isra'il and vv. 78-79; Ta Ha: 130; AnNur: 56-58; AI-'Ankabut: 45; Ar-Rum: 17-18, 31 and the E.N.'s thereof).
 

So be patient, [O Muhammad]. Indeed, the promise of Allah is truth. meaning in Urdu

پس اے نبیؐ، صبر کرو، اللہ کا وعدہ بر حق ہے، اپنے قصور کی معافی چاہو اور صبح و شام اپنے رب کی حمد کے ساتھ اس کی تسبیح کرتے رہو