Learn Spanish (Latin America): No puedes pasar ahora.

When you use poder (to be able to / can) with another verb, only the first verb is conjugated. Because puedes already tells us who is doing the action, the second verb stays in its dictionary form (the infinitive), which is pasar here.

Doesn't pasar mean 'to pass'? Why is it translated as 'come in'?

While pasar literally means 'to pass', it is the most common way to say 'come in' or 'enter' in everyday Spanish. You will often hear people say pasa to invite someone inside a room or house.

Why is no placed before puedes instead of between the two verbs?

In Spanish, the negative word no almost always goes directly in front of the conjugated verb. Even though there are two verbs working together here, the no must come before the main conjugated one (puedes) to make the statement negative.

Is it wrong to say tú no puedes pasar ahora?

It is perfectly fine to say that! However, Spanish speakers usually drop subject pronouns like tú because the verb ending in puedes already makes it crystal clear that 'you' are the one who cannot come in. Adding tú would just add extra emphasis.