smetnja

1. obstacle; hindrance
   Godine nisu smetnja da naučiš novi jezik. — Age is no obstacle to learning a new language.
2. (of a signal, connection) interference
   Izvini, ima smetnji na vezi, ne čujem te. — Sorry, there's interference on the line, I can't hear you.
3. (of health) disorder; problem
   Deca sa smetnjama u razvoju idu u ovu školu. — Children with developmental disorders attend this school.
   Otac ima srčane smetnje. — My father has heart problems.

In the senses of interference and health problems it is mostly plural: smetnje.