But to sit on my right hand and on my left hand is not my to give; but it shall be given to them for whom it is prepared.

but as to sitting at my right hand or at my left, that is not mine to give: it will be for those for whom it is reserved."

But to be seated at my right hand or at my left is not for me to give: but it is for those for whom it has been made ready.

but as to a seat at my right or at my left – that is not mine to give, but it is for those for whom it has been prepared."

Is not mine to give - See on Matthew 20:23 (note).

But to sit on my right hand and on my left, is mine to give,.... There being no such places in his kingdom in the sense they petitioned; and as for the glories of the heavenly state or eternal life, the gift of these was not to be settled now it being done already: and though he had a power to give yet only to them who were given him of his Father and who were ordained to such happiness as it follows:
but it shall be given to them for whom it is prepared; that is, of his Father as is expressed in Matthew 20:23 which is not to be understood as excluding these two persons but as including all others with them for whom the kingdom was prepared before the foundation of the world: the Ethiopic version therefore wrongly renders the words "but to sit on my right hand and on my left, I do not give to you, it is prepared for other": the Jew (g) very badly concludes from hence against the deity of Christ and his unity with the Father he not having power to do this; whereas Christ does not say he had no power to give this honour, but only describes the persons to whom he should give it; and these being persons for whom it, is prepared by his Father instead of destroying, proves their unity.
(g) R. Isaac, Chizzuk Emuna, par. 2. c. 20. p. 409.

But to sit on my right hand and on my left hand in not mine to give; but it shall be given to them for whom it is prepared--"of My Father" (Matthew 20:23). The supplement which our translators have inserted is approved by some good interpreters, and the proper sense of the word rendered "but" is certainly in favor of it. But besides that it makes the statement too elliptical--leaving too many words to be supplied--it seems to make our Lord repudiate the right to assign to each of His people his place in the kingdom of glory; a thing which He nowhere else does, but rather the contrary. It is true that He says their place is "prepared for them by His Father." But that is true of their admission to heaven at all; and yet from His great white throne Jesus will Himself adjudicate the kingdom, and authoritatively invite into it those on His right hand, calling them the "blessed of His Father"; so little inconsistency is there between the eternal choice of them by His Father, and that public adjudication of them, not only to heaven in general, but each to his own position in it, which all Scripture assigns to Christ. The true rendering, then, of this clause, we take it, is this: "But to sit on My right hand and on My left hand is not Mine to give, save to them for whom it is prepared." When therefore He says, "It is not Mine to give," the meaning is, "I cannot give it as a favor to whomsoever I please, or on a principle of favoritism; it belongs exclusively to those for whom it is prepared," &c. And if this be His meaning, it will be seen how far our Lord is from disclaiming the right to assign to each his proper place in His Kingdom; that on the contrary, He expressly asserts it, merely announcing that the principle of distribution is quite different from what these petitioners supposed. Our Lord, it will be observed, does not deny the petition of James and John, or say they shall not occupy the place in His kingdom which they now improperly sought:--for aught we know, that may be their true place. All we are sure of is, that their asking it was displeasing to Him "to whom all judgment is committed," and so was not fitted to gain their object, but just the reverse. (See what is taught in Luke 14:8-11). One at least of these brethren, as ALFORD strikingly remarks, saw on the right and on the left hand of their Lord, as He hung upon the tree, the crucified thieves; and bitter indeed must have been the remembrance of this ambitious prayer at that moment.

Save to them for whom it is prepared - Them who by patient continuance in well doing, seek for glory, and honour, and immortality. For these only eternal life is prepared. To these, only he will give it in that day; and to every man his own reward, according to his own labour.