piecewise defines a symbolic function, so you can't just give a double as input for the anonymous function. If you however explicitely make the substitution it will work
syms y
x = @(y) piecewise(y>2,1, 0);
subs(x,[1,2,3])
ans =
[ 0, 0, 1]
Although unless you really need it to be symbolic I would strongely recommend to remain working with numerical values:
x = @(y) double(y>2);
x([1,2,3])
ans =
0 0 1
