gstr1

yes it is a mandatory field.

but i already submitted my gstr1 without filling turnover for last FY mistakenly. . i didnt submit yet. as i will get audited statement for last FY only after 15th sept.

i mean i did submit but didnt didnt filed yet

After submit no change will occur... If u not fill turnover for last fy, then you couldn't add it

so what shall i do then??? please advise

wait for the revise return structure. If you also not find any revise data field, then leave it. If you receive show cause notice, gently reply them as it is a new law new structure, you have done mistakes, the intention was nothing to hide as you have paid due taxes to all authorities against your preceding Fy.
No need to worry much. We are human being mistakes can be done by mistake. I have noticed even GST official Twitter handler had also posted wrong information then they corrected later on.

Thank you so much