dsolve() by default tries to find find an explicit solution analytically i.e. y in terms of t.
In this case, it is unable to find that form and thus returns the warning and an empty symbolic array.
However, you can get the implicit form of the solution -
syms y(t)
ode = diff(y,t) == t^2/(1+y^2);
ySol = dsolve(ode,'Implicit', true)
