Incorrect use of '=' operator. To assign a value to a variable, use '='. To compare values for equality, use '=='. in column 20

close all;
dt = 0.464;%time step
u1=1;
u2=1;


k=0.12;
m=2.21;%initial parameters

v=1;
a=1.4;

tend=100;

t=0;
i=1

while t < tend-2*dt
    vh=v(i)-dt*k*(u1(i)-u2(i))/ (2*m);
 u1(i+1)-u2(1+1) = u1(i)-u2(i)+ dt*vh;
 a(i+1)= -k*u1(i+1)-u2(i)/2;
    v(i+1)= vh + dt*a(i+1)/2;
    i = i+1;
    t = t + dt
end

plot(linspace(0,tend,tend/dt),u1-u2);

[TADA]

What exactly are you trying to do here

u1(i+1)-u2(1+1) = u1(i)-u2(i)+ dt*vh;

You can't do an assignment into an arithmetic calculation