Undefined function 'variable' for input arguments of type char

when I run my cvx code on Linux system, it display this error :undefined function ‘variable’ for input arguments of type char
But on Windows system it does not display error.

% Objective update
cvx_begin quiet
variable Z(4,K) nonnegative
…
cvx_end

Any idea of why does this happen?

Try reinstalling CVX, and note whether there are any warnings or errors. If you are using CVX 3.0beta, switch to CVX 2.1. Beyond that, I have no idea.