Call errorbar with this syntax instead:
errorbar(x(1),y(1),SEM(1),'k','linestyle','none');
errorbar(x(2),y(2),SEM(2),'b','linestyle','none');
FYI, your code can be simplified to the following:
%Define the colors' RGB values in a matrix
col(1,:) = [1 0 0]; % -- red
col(2,:) = [0, 0.4470, 0.7410]; % -- blue
x = categorical({'Control', 'Folic Acid'}); %xaxis
x = reordercats(x,{'Control', 'Folic Acid'}); %specified order
y = [1.8493, 1.8]; %yaxis
SEM=[0.0168, 0.0170];
figure
hold on
b = bar(x, y, 0.5, 'FaceColor',col(2,:));
b.FaceColor = 'flat';
b.CData= col;
errorbar(x,y,SEM,'k','linestyle','none');
title('Brain Weight')
ylabel('Weight (g)')
hold off
You only need one call each for bar and errorbar, and the multiple calls to hold on are extraneous.
