since the partnership return was terminated in June 2019, the final return was due by 9/15/2019 2.5 months after the June year-end.  your choices are to pay the penalty or ask for a waiver (penalty not interest) claiming lack of knowledge and first time filing return late (if late before just pay).  it's up to the IRS to make a decision as to whether or not they will honor the request.  

you use the 2018 tax forms if the 2019 forms are not out and specify a year beginning 1/1/19 and ending 6/30/2019 9 or whatever the actual date was.