1 - You have not defined what Ld and x are, so I have assumed them to be symbolic variables.
2 - use n1 as the loop index instead of 1:n1, as n1 is an array and not a scalar.
3 - Ncr and y are not used in the code. Bring Ncr22 and NcrN out of the loop as they are not varying with the loop.
4 - Do not use "int" as a variable name as it is a function in MATLAB.
5 - There is no guarantee that any of the root obtained, assigned to int0, will be real. The roots obtained are in terms of Ld ,which is a symbolic variable. If Ld = -1, then all roots will be imaginary. Additionally, inter can be an array as well, which you can not put as limits to plot the function in.
It's not clear which function from the code is supposed to be the red curve in the image above. And the red curve doesn't cross the x-axis in the image, so it's not clear what your title is supposed to be.
syms z L EI kd N n c d x Ld
Ncr1 =(2.4674*EI*(2*n - 1)^2)/L^2;
Ncr2 =-(0.0205*L^3*kd*(39.4784*n + 2*cos(6.2832*n) - 39.4784*n^2 - 7.8696))/(d*(2*n - 1)^4);
n1=1:10;
Ncr22=subs(Ncr1,n,n1)+subs(Ncr2,n,n1);
NcrN=subs(Ncr22,[L EI d L kd],[1 1/x^2/4 1/Ld 1 x^2]);
for n=1%n1
%Ncr=Ncr22(n)
int0=solve(NcrN(n+1)-NcrN(n)==0,x)
%inter=int0(real(int0)>0&imag(int0)==0);
%y=subs(NcrN(n),x,inter)
%fplot(x,NcrN(n),[0 inter],'r','LineWidth',1.5)
end
%fplot(NcrN.','--');
