GSTR-1 Rectification

Suppose one RTP has GSTR-1 and GSTR- 3B mismatch in F.Y 2018-19.He rectified the difference in June19 GSTR-1 by submitting negative figure return.But he rectified it or submitted GSTR-1 in December 20.If it is not allowed by tax official.Can he rectify the tax liability of 2018-19 in his annual return of 2018-19,the last date of which is extended upto March 21? Can anyone who otherwise not bound to file annual return,can file it for rectification of tax figure only?