The error message is clear: join_function.m is a script file and then it does not accept input arguments. Only functions do this. See https://www.mathworks.com/help/matlab/matlab_prog/scripts-and-functions.html and https://www.mathworks.com/help/matlab/learn_matlab/scripts-and-functions.html .
The code has more problems. sprintf('data{1,%d}', j) creates strings, char vectors to be exact. But in the joining you try to access the contents of an data array. This is something completely different. 'data{1,2}' is a char vector, but does not carry any information about the array called "data".
It is strange, that you create 'data{1,1}' in the loop, but try to access the two elements of the cell array data{1,1} and data{1,2} inside the outerjoin function.
I guess, you want something like this:
numfiles = 100;
demand = table();
for j = 1:numfiles
demand = outerjoin(demand, data{1, j}, 'Keys', 'time', 'MergeKeys', true);
end
