The function fileparts creates output with the same data type as the input.
% String input = string output
fileS = "H:/user4/matlab/myfile.txt";
[filepath,name,ext] = fileparts(fileS)
% Char array input = char array output
fileC = 'H:/user4/matlab/myfile.txt';
[filepath,name,ext] = fileparts(fileC)
Check what you are using for input. You could use the convertStringsToChars function to convert your input to fileparts to char if necessary.
[filepath,name,ext] = fileparts(convertStringsToChars(fileS))
Or simply char(string), as Fangjun showed
[filepath,name,ext] = fileparts(char(fileS))
