Having "Retirement plan" checked in box 13 of your W-2 might require an amended return.
On the tax return that you filed, do you have an IRA deduction on Schedule 1 line 20? If not, if Schedule 1 line 20 is blank or you do not have Schedule 1 at all, then checking "Retirement plan" will not affect your tax return, so there is nothing to amend it for.
If you do have an IRA deduction on Schedule 1 line 20, then depending on your filing status and your Adjusted Gross Income (AGI), checking "Retirement plan" might reduce or eliminate your IRA deduction. In that case, you would definitely have to file an amended return. Since you have already made the IRA contribution, if your IRA deduction is reduced or eliminated that means that part or all of your IRA contribution becomes nondeductible. So in addition to reducing or eliminating the deduction, you would have to include Form 8606 in your amended return.
