I don't have the psychotoolbox, but have a look at the documentation of kbCheck.
The reason you get an error is because the keyCode it returns is either empty or more than one value.
If it can be empty, you could change your test to
if ~isempty(keyCode) && ismember(kbName(keyCode), {'m', 'z'})
If it can be more than one value then
if any(ismember(kbName(keyCode), {'m', 'z'}))
or
if all(ismember(kbName(keyCode), {'m', 'z'}))
depending on what you want to test.
Note that
if ismember(kbName(keyCode), {'m', 'z'})
is equivalent to
if strcmp(KbName(keyCode), 'm') || strcmp(KbName(keyCode), 'z')
but obviously less to type and more extensible if you want to add more accepted key codes.
