When 이/가 particle is added to form -ㄹ/을 수가 있다/없다 it has the same meaning but adds a slight emphasis that you cannot do sth.
Much like this, the 은/는 particle can be added to imply a contrast, forming -ㄹ/을 수는 있다/없다. For example, 잘 수는 없어요 – I can’t sleep here (but I may be able to sleep somewhere else).
However, you cannot put 을/를 particle in this construction, like -ㄹ/을 수를 있다 (X)