The cause:

fopen is fail because MATLAB is not permitted to read/write on the file you have specified. Therefore, the error appears.

Solution: 

This error will disappear if fopen is added permissions. You can see in  http://www.mathworks.se/help/matlab/ref/fopen.html , the fopen syntax is:

fileID = fopen(filename,permission)

‘r’ (default) | ‘w’ | ‘a’ | ‘r+’ | ‘w+’ | ‘a+’ | … are some possible permissions and and below is their functions:

‘r’ – Opening a file for reading.

‘w’ – Opening or creating a new file for writing. Cutting the existing contents, if have.

‘a’ – Opening or creating a new file for writing. Adding data to the end of the file.

‘r+’ – Opening file for reading and writing.

‘w+’ – Opening or creating new file for reading and writing. Cutting existing contents, if have.

‘a+’ – Opening or creating new file for reading and writing. Adding data to the end of the file.

…

If fopen is use with no permissions, or I use ‘r’ (default) option, fopen will return -1 value. It is an error. Then I use below commands and they give me success:

fid=fopen('tmp.txt', 'w'); 
fid=fopen('tmp.txt', 'a');