First, April 15, 2021 represents two deadlines:
1. to file your 1040, and
2. to pay any tax due
So you have already filed your return? You can file the amended return (i.e., the paperwork) any time in the next three years.
You are probably more concerned about the owing money after April 15th. First, the 1040X is highly likely to be ready in time for you to amend the return and make any payment by April 15th.
Second, so long as you have an idea of how much additional tax you may owe, you can pay the IRS anytime, even without a return.
Please see this IRS webpage for various ways you can pay the IRS. Note that your payment will be tied to your account by your name and Social Security number, which is why you don't have to wait until you have a tax return to go with the payment.
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