Can a proprietary firm into llp

File the return for AY-2020-21 through AO.

Form LLP first & transfer all assets & liabilities using sale consideration

Condonation of delay under section 119(2)(b).
complete process is very important. Obviously involment of AO is there.
But it will take time.Then convert it to llp.

Condonation of delay for AY-2020-21 can't be filed for e-filing of ITR.

@ Aayesha

You can Form LLP first as suggested ,and can also transfer all assets to LLP ( Like we call it transfer of asset ) while it be in the form of your contribution ,For Income Tax purpose you have to file ITR ,and it's your default ! In eye of law you and proprietorship is not different ...Non Filing of ITR will not impact unless otherwise you are company(Pvt) or Registered partnership...