You should have define
public void blurImage(int numbertodivide)
instead of public void blurImage(intwger numbertodivide)

and you do not need
integer rows;
integer columns;
numbertodivide = rows*columns;
in the lib panel.

There are other errors: for example you call it with blurImage(); without any parameter.

I corrected several error in your code (see attached file).
However, you are not consistant. for exmaple:yo have
  java.awt.image.BufferedImageOp op = new java.awt.image.ConvolveOp( new java.awt.image.Kernel(rows, columns, matrix) );
which involve rows and columns. However, it is not related to numbertodivide in your code, and the setting of matrix.

You should modify the following code to make it more consistant.
float[] matrix = new float[numbertodivide];
for (int i = 0; i < numbertodivide; i++)
matrix[i] = 1.0f/numbertodivide*1.0f;