The FName output in your uigetfile call is a cell array. When you use parentheses to index into a cell array the result is also a cell array. Calling strcat on a cell array and a char array results in a cell array. fopen does not accept a cell array as its first input.
Instead use curly braces to extract the contents of the cell in FName, which will be a char array. Calling strcat on a char array and a char array results in a char array that fopen can accept.
sample = {'abc'; 'def'; 'ghi'}
xCell = sample(1)
xChar = sample{1}
yCell = strcat(xCell, '123')
yChar = strcat(xChar, '123')
whos
