How can one reconcile the Hadith, "None of you should say: 'O Allah, forgive me if You wish; O Allah, have mercy on me if You wish,' but rather be firm in supplication, for Allah cannot be compelled," with the Hadith, "No harm, it is a purification, if Allah wills"?

The Prophet, may Allah bless him and grant him peace, forbade attaching "in sha Allah" (God willing) to supplications. He commanded the supplicant to be firm in their request and to express a great desire to Allah. This is because attaching "in sha Allah" to a supplication suggests that the supplicant is not in need of what they are asking for, and it implies that Allah might do something against His will, whereas He, glory be to Him, has no one to compel Him. As for the Prophet's saying to the sick person, "(It's) no harm, a purification, if Allah wills," this is not of that category. He said it as an expression of hope, seeking blessings, and entrusting the matter to Allah, for not every illness purifies from sins.