Use `Select by Expression` from the attribute table to select polygons of "MUNICIPIO"=5 and use Fixed Distance Buffer for the selected polygons. 

Use Fixed Distance Buffer from SAGA tools which you can find it from `Processing toolbox -> SAGA -> Vector General tools -> Fixed Distance Buffer`. It should work.

Make sure that Use only selected features option is checked from the Processing Option. It should be checked by default.

[![enter image description here][1]][1]


  [1]: https://i.sstatic.net/f0w94.png