Try
TheData = cssm();
where
function TheData = cssm()
FormatStr = repmat( '%f', 1,12 );
FileId = fopen('b.txt');
DataCell = textscan(FileId, FormatStr ...
, 'HeaderLines', 39, 'CollectOutput', true );
fclose(FileId);
TheData = DataCell{1};
end
The problem with your code has something to do with the delimiter being "one space" or "one or many spaces". And no need (/better not) to include space in the format-string. Default takes care of it. See the documentation.
