I do my sister's taxes. She moved from MD to FL in 2021. What address do I put on here 2020 IRS and MD tax forms? her old MD addreess? or her new FL address?

If she lives in FL now use her current address in FL so that if she needs to receive mail from the IRS or MD it will go to her instead of going to her old address in MD.    You can still indicate within the software that she lived in MD in 2020 because she will need to file a MD return for 2020.