I have a question that am stuck with. I filed my return for FY 2019-20 (AY 2020-21) last year. There was a net taxable amount I had to pay, which I did before filing the return. While paying that online, I paid the challan under the Minor Head “300 - Self Assessment Tax, instead of the Minor Head “400 - Tax on Regular Assessment. This reflects in my 26AS for the correct AY.

My IT return got processed yesterday and I just got a notification of 143 (1), where this amount was NOT considered as paid, where the reason quoted is “Challan is not available for the AY”. I wanted to understand two things: 

1. Could paying the amount under 300 - self assessment category be the reason for this? If so, what is the procedure for changing the minor head on the challan? On the tax website, it says to contact the Assessing Officer. If so, how do I reach that person? 

2. If it is not the reason, what else could be the reason for mismatch? 

3. Should I just go ahead the file a rectified return with the same details again? 

Please advise on the next steps.