I would talk to them about it, see if there is a reason they feel that way. Sometimes people need to be reminded that other people should get the chance to be those other positions; a well-rounded partner is a huge benefit to everyone. You could ask other shifts at your store about it and get their feedback on how they handle it. Last resort, you could talk to the store manager about it.

You have 2 ways you can go about this:

1. Pull them aside and talk to them about why they only want to do those positions or why they don’t feel comfortable about other positions. Once they tell you the reason (doesn’t matter what it is) acknowledge them and make sure they know that you listened and care about it. (Since that partner gives you attitude I’m assuming they just don’t like other positions. If they don’t feel comfortable in other positions, tell them that you will give them practice and coaching in those positions).
Remind them that YOU are the shift and they are a barista and you have a job to do aka run the floor. Their job is to do whatever position you need them at. Tell them how you need to rotate partners and that every partner needs to be good at any position.
If the problem continues, talk to your SM and do another meeting with the partner, but make sure either SM is present or at least another shift. You gotta have a witness. If the problem continues, tell your SM and let the SM deal with it. Make sure to document everything (every single conversation you have with said partner, the attitude, when it happened, time date location. Take notes of everything. Also see if any other partner witnessed that partner giving you attitude.

2. Or you can be petty. Never put them on bar or DTO. Put them in a window, have them clean the floors and drains, etc. if they give you attitude, give it back and remind them who is the boss.

Doing option one is better choice though.