You may sit in a chair of course. May I suggest sitting towards the edge of the chair, with your back straight, so you don’t become too relaxed and fall asleep.
You don’t need to get into the lotus position at all, or any position/posture that creates aches and pain, as long as you are in a posture/position from which you are able to form a comfortable observation of the rising and falling of the abdomen, with your back as straight as can be.
Meditation practice allows one to deal with aches and pain, head on, by observing the nature of this pain, rather than judging it (like/dislike), and this will help alleviate the pain in the long run. A meditation session may be very pleasant, but it won’t cultivate an understanding of the nature of this pain if you cling onto the feeling.