The negative of мћжно 'one may, one can' is нельзї 'one may not, one cannot'. (The usual way of forming the negative with не is not possible with the word мћжно.) Use the Imperfective aspect with нельзї, unless you're talking about the impossibility of getting something done on a specific occasion:
Impf. Тџт нельзї переходќть џлицу. You can't cross the street here. (No jaywalking!)
Pf. Сейчђс нельзї перейтќ џлицу. You can't cross the street now. (There's too much traffic.)
Нельзї and мћжно are predicative words; use the Dative case with them if you want to specify the person involved.
Врђч сказђл, что мнљ нельзї вставђть. The doctor said I can't get up. (I must stay in bed.)