f is a vector, as well as W. So you must use the dotted version of the division (that is ./ that performs the operation elementwise). You may find more information here
https://www.mathworks.com/help/matlab/matlab_prog/array-vs-matrix-operations.html
clear;
clear all;
% plotting frequency-current graph according to variable frequency values
f=100:100:100000; % variable frequency values
V=3.536; % AC voltage source
L=33*10^-3; %inductor (henry)
C=47*10^-9; %capacitor (farad)
R=820; %resistor (ohm)
W=2*(pi)*f; % Angular frequency formula
XL=W*L; % inductor impedance
XC=1./(W*C); % capacitor impedance
Z=(2*R)+1i*(XL-XC); % total impedance
I=V./Z; % current
figure,plot(f,I)
Note that you are plotting only the real part, as noticed in the warning
