The condition for interferences,: two waves created constructively interfere if their optical path differ by a multiple of the wavelength, that is introduced in elementary optic course is necessary but not sufficient.
A light source emits wave trains that have length and duration. The interferences occur only occur if the interferometer splits each wave train into two beams (creating two artificial coherent light sources), make the two beams travel a different path and reunite them on the screen. Two different light sources or two different wave trains emitted by he same source cannot interfere.
You can learn more about coherence length and time there.