I guess this variable save_var{i} in the 4th last line of your code is returning a cell. Whereas, the function save expects a string scalar or a character vector.
Try casting save_var{i} to char.
sv_direct ={'D:\One drive\OneDrive - XUniversity\PhD\TecPlot\MATLAB_sim_var\'};
coupled_data_export = num2cell(rand(3));
save_var{i} = {'coupled_data_export'};
for i = 1:length(sv_direct)
compare_locs{i} = sprintf('%s%s_subplot_figures/%s/%s/%s', sv_direct{i},string(folder_pts(5)),string(coupled_FN(1)),currDate,string(folder_pts(6)));
% Finally, create the folder if it doesn't exist already.
if ~exist(compare_locs{i}, 'dir')
mkdir(compare_locs{i});
end
end
for i = 1:length(sv_direct)
save_locs{i} = sprintf('%sEqualised_for_TP_export/%s/%s/%s', sv_direct{i},string(coupled_FN(1)),currDate,string(folder_pts(6)));
if ~exist(save_locs{i}, 'dir')
mkdir(save_locs{i});
end
end
for i = 1:length(save_var)
save_name{i} = char(save_var{i});
for k = 1 : length(save_locs)
baseFileName = char(sprintf('%s.mat', save_name{i}));
fullMatFileName = fullfile(save_locs{k}, baseFileName);
save(fullMatFileName, char(save_var{i}));
fprintf('Saving to: %s.\n', save_locs{k});
end
end
