documentclass[a4paper,12pt]{report}
usepackage[T1]{fontenc}
usepackage[round]{natbib}
usepackage[spanish]{babel}
usepackage[utf8]{inputenc}
begin{document}
citet{DIJKSTRA1959}
bibliographystyle{plainnat}
bibliography{mybib}
end{document}

@article{DIJKSTRA1959,
   author    = {Dijkstra, E. W.},
   title     = {A note on two problems in connection with graphs},
   journal   = {Numer. Math.},
   pages     = {269-271}
   year      = {1959},
   volume    = 1
}

pdflatex file
bibtex file

The style file: plainnat.bst
Database file #1: mybib.bib
I was expecting a `,' or a `}'---line 6 of file mybib.bib
 :
 :    year      = {1959},
(Error may have been on previous line)
I'm skipping whatever remains of this entry
Warning--empty year in DIJKSTRA1959
Warning--empty year in DIJKSTRA1959

bibitem[Dijkstra()]{DIJKSTRA1959}
E.~W. Dijkstra.
newblock A note on two problems in connection with graphs.
newblock emph{Numer. Math.}, pages 269--271.

@article{DIJKSTRA1959,
   author    = {Dijkstra, E. W.},
   title     = {A note on two problems in connection with graphs},
   journal   = {Numer. Math.},
   pages     = {269-271},
   year      = {1959},
   volume    = 1
}

bibitem[Dijkstra(1959)]{DIJKSTRA1959}
E.~W. Dijkstra.
newblock A note on two problems in connection with graphs.
newblock emph{Numer. Math.}, 1:penalty0 269--271, 1959.