Taking your question exactly as written, you were in the process of filing for divorce from your husband but he died before the divorce was final. You filed a married filing joint return (which is allowed in that situation). Your husband was making support payments to you, either voluntary or as part of a court ordered separation pending divorce.
In that case, you can't deduct his alimony payments to you as a deduction on your joint tax return. You also don't report his alimony payments as income. You simply ignore them. You would also ignore child support payments.
If you deducted alimony your spouse paid but did not report it as your income, then you owe the tax the IRS has assessed.
If you deducted alimony your spouse paid AND reported the alimony as your income, then the IRS has identified one mistake but not the other. You may owe less tax than the IRS assessed, and you may even owe no tax at all. But you will need to prepare an amended return to remove the alimony both ways, calculate any tax owed, and send the amended return and a letter of explanation to the IRS at the address given in their letter to you.
