Why does it show that I owe additional taxes on my RMD Vanguard withdrawal when the was calculated by Vanguard?

[MinhT1]

Tax withheld by Vanguard is just an estimated advance payment of your taxes.

The final tax amount can only be determined when you fill out your tax return. The taxes withheld serve as a credit against your tax liability. If too much tax was withheld, you'll receive a refund; otherwise, there'll be a tax due.