syms can't be used

[madhan ravi]

>> which syms -all
/Applications/MATLAB_R2018b.app/toolbox/symbolic/symbolic/syms.m
>> 

If you have anything other than that it might be the issue. I guess it's due to MUPAD.

[洋 于]

and then？

[Stephan]

Would be worth a try to update your Matlab - maybe problem is solved then.

[Star Strider]

The MATLAB version is likely not the problem.

Consider that you are requesting a (1x0) scalar, when scalars are defined as (1x1) in MATLAB. With any dimension defined as 0, the result is an empty vector.

[John D'Errico]

You are trying to use syms, in the most basic way possible.

syms x
>> x
x =
x

And that is what SHOULD happen. What you show is a failure in the simplest thing you can do.

So often when I see a problem like this, it means the person has inadvertantly created a function of their own, with a colliding name. Perhaps you created an m-file named sym or syms, which effectively overloads the existing tools. Your function now gets called in place of the correct function. Or, perhaps you mistakenly edited the existing m-files from MATLAB. People do this by mistake, and then mistakenly save the result. These are all bad things to do.

So the very first thing I would try is to redownload the symbolic toolbox. Even better would be to start from a fresh copy of MATLAB. That is cleaner yet.