Hi,
dont worry,
section 143(1a) is just claim for outstanding demand and section 245 is for adjusting any refund with the outstanding demand.
i need few details-
this demand is related to which year.
you need to check by login to IT site and pending action from your site there you can upload return under section 143 (1)
but i will advice you better take any Chartered Accountant services, and these sites are only for filing basic returns of salaried employees
NEVER RELY ON THESE SITES
if you share your details with me i can help you in filing the return
my email id is agarwalvishakha23@gmail.com