Itr rectification

Hello Members,

I filed ITR and got intimation u/s 143(1). I found that i had made mistake while filing ITR. There is TDS in Form 26AS from two firms, i included that in ITR but the amount of TDS mentioned against each firm got interchanged. Because of that my refund amount is reduced by half. Can i file rectitifcation to correct the mistake and get the remining amount refunded. If yes, When i can file the rectificatiton?

Also AY18-19 is not appearing in rectification request, can anybody suggest it is problem in the site or is specific to me.

Thanks