Tax demand because CPC could not find self assessment payment

If you've received a tax demand because the CPC couldn't locate your self-assessment payment, despite you having paid in full and entered the challan details correctly, you should first verify your records. Check Form 26AS and your challan copy for any errors in the assessment year or PAN. If everything is in order on your end, you can dispute the demand online by selecting an appropriate reason like 'Demand paid and Challan has CIN' and uploading your supporting documents.