Filing rectification request after 8 years

I filed ITR-4S of my senior citizen mother in AY2013-14 but due to mistake, i filed some of the TDS under TCS heading.

This caused a mismatch of the tax deducted.

So I filed rectification request next year but the same was not accepted by IT dept.

And I am still getting demand notices from IT dept as they still show that the TDS amount is not matching.

What is the best way to reply to the demand notice after 8 years have passed?

I am unable to e-file recitification request now: Not clear if its due to the portal error or due to any rule of not allowing rectification of ITR after 4 years.

Thanks