The second stimulus check was supposed to be included in 2020, even though you received it in 2021. Read the earlier replies above. The IRS is right.
The IRA custodian is the bank, brokerage, or other financial institution that holds the IRA funds and manages your IRA account.
It doesn't matter when you received the money from the IRA. What matters is when it was distributed from the IRA. If you took it out late in the year, it could have been distributed in December but you didn't get it until January. (Or you could be mistaken about when you got it. You said "I think I received the money in 2021.")
If the IRA custodian reported the distribution in 2020, it has to go on your 2020 tax return. If you got a 2020 Form 1099-R for it, it goes on your 2020 tax return.
