If you filed in March, then you return should be processed by now.
You can go to your IRS account and see if it shows the balance due. You will also be able to see if they did send you any letter that may have got lost in the mail.
If you have a balance, you should be able to apply for the payment plan. However, you can apply for a payment plan prior to them processing and seeing your balance.
To request a long-term payment plan, use the Online Payment Agreement application. Even if the IRS hasn’t yet issued you a bill, you may establish a pre-assessed agreement by entering the balance you’ll owe from your tax return. An online payment agreement is quick and has a lower user fee compared to other application methods.
If you are being rejected, maybe try a different computer to see if it is a tech issue? Otherwise, the IRS is the only one who can tell you what is happening.
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