MATLAB: “syms” command does not respond in MATLAB 2018a

symbolicSymbolic Math Toolboxtoolbox

when I try "syms x y", it keeps matlab in the busy status and never finishes the execution.
I am using MATLAB 2018a, student version with license. I used "which" and "ver" to confirm that the symbolic toolbox has already been installed. And I tried to reboot my laptop but not working.
Could anyone help me to solve this situation? Thanks!