Learn Russian: Нельзя постоянно отдыхать.

We use the imperfective here for two reasons. First, the word постоянно (constantly, all the time) tells us this is an ongoing or repeated process, not a single finished result. Second, when нельзя means 'you shouldn't' or 'it is a bad idea', it almost always takes an imperfective verb.

Wait, doesn't нельзя mean 'it is impossible'? Why is it translated as 'You can't'?

Нельзя can mean both 'impossible' and 'not allowed' or 'one shouldn't'. When paired with an imperfective verb like отдыхать, it usually expresses prohibition or strong advice not to do something. This sentence is impersonal, meaning there is no specific 'you' in the Russian version, but in English, we naturally translate this general rule as 'You can't' or 'One shouldn't'.

What would it mean if we used a perfective verb after нельзя?

If you use a perfective verb after нельзя, the meaning usually shifts from 'you shouldn't do it' to 'it is physically impossible to achieve the result'. For example, if you used a perfective verb here, it would mean 'it is impossible to successfully finish resting', which doesn't make much sense. Since we want to say 'you shouldn't be in the constant state of resting', the imperfective отдыхать is the correct choice.