In case of Income Tax return, if an income is undisclosed the same should be shown vide a revised return, however the due date for FY 2019-20 has passed and the same cannot be revised,
You may write to your AO highlighting the same and seek for manual revision.
The last option definitely is disclosing the same in pursuing ITR.
In case of GST Return, please note exports can be done without payment of GST provided LUT is filed and the details are disclosed in GSTR. In GST there is no concept if revision and therefore it has to be shown in current return only. You may show the said difference in current month GSTR filing and in 2019-20 annual filing, the difference would arise which can be shown as belated disclosure in t=GSTR in 21-22 and accordingly be reconciled.
However, please note that since no LUT was provided then, you run a risk of notice by GST dept. seeking tax payment on such exports, although your case in this matter would be defendable.
Lastly please note, your classifying as a clerical error will not provide you any relief, as any taxpayer whether bonafide or not always seeks the plea of clerical error only. The additional time limit is provided for rectifying such mistakes only, which in your case has expired.
Let's hope for the best.
Please advise in case of any further clarification.
Thanks & Regards,
CA Aditya Dhanuka.