To understand the problem here, you need to understand how cell arrays work and how for loops work.
The for loop in the example works roughly the same as the following:
for index = 1:numel(column_file_number)
i = column_file_number(index);
There are two ways of accessing the entries of a cell array:
1) curly brackets: column_file_number{index} will give you the element in location specified by index. If index is a vector containing several indices column_file_number{index} will return a comma separated list of the entries at index in the cell array. To retrieve the elements do something like [v1,v2] = column_file_number{[1,2]}.
2) round brackets: column_file_number(index) will give you a cell array containing the elements of column_file_number at the indices specified by index - even if index only contains one element!
In the example the for loop uses method 2 - that is the Name_file variable you get is a cell array of one element instead of a char element. So to solve the problem you can change line 5 to data(:,end+1)=importdata(Name_file{1}); or change line 3 to Name_file= ['Degree210B', i{1}, '.dat'];.
The problem is a very simple problem, but I think it is quite common (I remember having the same problem when I first learned matlab), and understanding why the problem occurs and how to solve it is important to get fluent in matlab!
char:importdata(char(Name_file)).