Arrays have incompatible sizes for this operation.

OptSelection = input(' Will you like to Add Delete ','s');
while strcmpi(OptSelection,"Add") ~=1 && strcmp(OptSelection,"Delete") ~=1
    
    fprintf(' You have selected a not existing option \n');
    fprintf(' Make sure that the initials of the selected option are capital \n');
    OptSelection = input(' Selected the correct option ','s');
end
fprintf('Print %s \n',OptSelection);
if OptSelection == 'Delete'
    fprintf('Okay1');
else OptSelection == 'Add';
    fprintf('Okay2');
end

Answer 1

[Walter Roberson]

while strcmpi(OptSelection,"Add") ~=1 && strcmp(OptSelection,"Delete") ~=1

That is valid code, if a bit awkward. Less awkward would be something like

while ~ismember(lower(OptSeletion), ["add", "delete"])

then

    if OptSelection == 'Delete'

Earlier you used "Delete" instead of 'Delete' and you used strcmpi() instead of == . The previous test was valid. But this test is comparing the character vector in OptSelection to the character vector 'Delete'. This is like coding if [65 100 100] == [68 101 108 101 116 101] -- you know it is going to error out because the two vectors are not the same length.

When you are comparing character vectors use strcmp() or strcmpi() .

Note that "Delete" is not a character vector, it is a scalar string() object. The == operation is defined for string objects and automatically takes different lengths into account; furthermore if you have a character vector == a string object, the character vector is automatically converted to a string object. So it would be valid to code

    if OptSelection == "Delete"

... or you can use strcmp() or strcmpi()

Note that

         fprintf(' Make sure that the initials of the selected option are capital \n');

the restriction to capitals is not necessary if you use strcmpi()

Have you considered using questdlg ?