Need help! underachiever in team

So i was previous manager for a small team of blue collar workers. I got recently promoted to an administrative team. Team is split up per product line. One person has come back from 4 month sickness, is an underachiever and undermining the other team member. she said dont want to do certain tasks cause 'too stressy for me' but HR and Director: no adaptation of job for med reason in the company. i distributed task list, she has less tasks then the other woman,but even then she dont finish all the tasks. some are important like confirming new contracts in system.

i have weekly meeting with her and the other woman , and its not going well. she just says, too bad i couldnt do my task. the other employee tries to overcompensate and finish task in her place which means she is very overworked.

the undeachiever is 55 years old. she told me she dont like the weekly planning and meeting.

how can i handle this? the rest of the department is going wrll