Your .mat file contains a 3x2 cell array, each cell of which contains a 1x9 numeric array: this data cannot be saved in a 2D array file format such as a text file or an Excel file. To save in a 2D array file format (i.e. a text file or Excel file) you will need to have a cell array containing scalar values, or use a numeric array, or use some format that encodes nested data (this is not trivial). Here I used cell2mat to form one 3x18 numeric array, which can easily be saved in a text file or an Excel file:
>> S = load('20-8-2018.mat');
>> M = cell2mat(S.feature_vec);
>> csvwrite('output.dat',M) % to make a text file
>> xlswrite('output.xls',M) % to make an Excel file
