None of you should say: O Allah, forgive me if You will, have mercy on me if You will, and grant me sustenance if You will. Let him be determined in his request, for indeed, He does what He wills; none can compel Him

The Prophet (may Allah's peace and blessings be upon him) forbade making one's supplication conditional on something, even if it is Allah's will, as it is a well-known certainty that He does not forgive unless He wills. There is no point in conditioning one's supplication on His will, as this condition is only relevant for someone who may do something against his will under compulsion, which is something that Allah is far exalted above.

The Prophet (may Allah's peace and blessings be upon him) clarified this at the end of the Hadīth by saying that, indeed, none can compel Him. Moreover, nothing is too great for Allah to give; He is not incapable of anything, nor is there anything too great for Him that one would say: if You will. Making supplication conditional on His will is a form of being self-sufficient regarding His forgiveness. The statement: "If You will to give me such-and-such, then do so"

is only used when addressing someone whom one does not need or someone incapable. However, when addressing the Able Whom one needs, then one should be determined in his request, asking for his needs and imploring Allah and resorting to Him, as He is the Self-Sufficient, the Perfect, and the One Capable of all things.