The error message likely arises from specifying the membership function parameters as a vector of length two, [0, 10]. However, the parameter of a 'constant'-type singleton membership function must be scalar, as demonstrated in the example below.
%% Fuzzy System
fis = sugfis;
% Fuzzy Input 1
fis = addInput(fis, [-1 +1], 'Name', 'x', 'NumMFs', 4, 'MFType', "gaussmf");
% Fuzzy Output
fis = addOutput(fis, [-1 +1], 'Name', 'y');
% Staircase
fis = addMF(fis, 'y', 'constant', -1.0, 'Name', 'out1');
fis = addMF(fis, 'y', 'constant', -1/3, 'Name', 'out2');
fis = addMF(fis, 'y', 'constant', 1/3, 'Name', 'out3');
fis = addMF(fis, 'y', 'constant', 1.0, 'Name', 'out4');
% Fuzzy Rules
rules = [
"x==mf1 => y=out1"
"x==mf2 => y=out2"
"x==mf3 => y=out3"
"x==mf4 => y=out4"
];
fis = addRule(fis, rules);
figure
plotmf(fis, 'input', 1, 201), grid on, title('Input fuzzy sets')
figure
opt = gensurfOptions('NumGridPoints', 201);
gensurf(fis, opt), title('Fuzzy system output')
