If you are self- employed, you had the option to defer half of your SE tax and it is almost due. You would have received a letter from the IRS telling you to pay that remainder recently. Otherwise, we will need more information.
Is 309 the tax due on your tax return line 37? You do owe that when you filed. Did you have self employment income? You might still owe self employment tax on that.
The self employment tax deferral was an optional benefit. It is also a deferral rather than forgiveness, so those amounts will eventually need to be repaid. Half of the deferred amount is due on December 31, 2021, and the other half is due on December 31, 2022.
Check if you have a Schedule 3 line 12e which goes to 1040 line 31.
How to pay the deferred self employment tax
https://www.irs.gov/newsroom/how-self-employed-individuals-and-household-employers-repay-deferred-social-security-tax
The payment date for deferred taxes has been extended to January 3, 2022 for the first half and January 3, 2023 for the second half.
You are correct that you can't amend your current payment plan for income taxes; however, individuals who are unable to pay the full deferred tax amount should pay whatever they are able to pay by the installment due dates to limit penalty and interest charges.
If the installment amount is not paid in full, IRS will send the taxpayer a balance due notice. Taxpayers should follow instructions on the notice to make a payment or apply for a payment plan. They can also visit the Paying Your Taxes page on IRS.gov for additional information about ways they can pay, what to do when they can't pay, and viewing their tax account.
Click this link for more detailed info on Deferred SE Tax Payments.