I think you are using the terms string and char array interchangably, but they are two separate data types in MATLAB. Strings and shown with double quotes while char arrays use single quotes ("MgBr2" vs 'MgBr2'). When combining chars, you have to take into consideration their length, as each character is placed in its own column. Strings only use a single column
For this reason, my preference would be to use strings. You can convert chars to strings using the string function.
the_best_salt_1 = string(the_best_salt_1);
the_best_salt_1(nr_zones_analyzed) = string(best_salt_1);
Otherwise, you will need to take into consideration the legnth of your char, meaning your assignment must include row and column indices.
the_best_salt_1='test';
nr_zones_analyzed = 2;
best_salt_1 = 'MgBr2';
% this works
the_best_salt_1(nr_zones_analyzed,1:length(best_salt_1)) = best_salt_1
% Your error
the_best_salt_1(nr_zones_analyzed+1)=best_salt_1
