Reply to Income Tax demand notice u/s 143(1)

I have received income tax demand notice u/s 143(1) in response to revised return I filed for AY 2020-21. The tax demand is due to mismatch of self-assessment tax paid receipt challan number mentioned in ITR (4 digit number : 1422) and what is shows in Form-26AS (5 digit number : 01422). I want to file rectification in reply to this notice, but I am not as still on new Income tax website, rectification option is not available. In this context, I have below questions. Please answer them and guide me. I would appreciate detailed reply.

1 - How many days I have to reply to tax demand notice?
2 - Should I wait to reply the tax demand notice till rectification option is made available on new Income tax website and I file rectification?
3 - In case I have to give reply to tax demand notice in certain time limit, then what kind of reply I should give as there is no rectification option available on new Income Tax website?

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