Error using Xls write

Error using xlswrite (line 219) Invoke Error, Dispatch Exception: Source: Microsoft Excel Description: Microsoft Excel cannot access the file 'C:\Program Files\MATLAB\MATLAB Production Server\R2015a\bin\7136D000'. There are several possible reasons:

• The file name or path does not exist. • The file is being used by another program. • The workbook you are trying to save has the same name as a currently open workbook.

This is my code :

clear all clc n=input('Enter the number of cases do you want to analyze : ' ); for num = 1 : n [u1,u2,u3,t,dx,dt,a,x]=inputdata; if num==1 figure plot(u1(:,1),'k'); xlabel(' \itX \rightarrow '); ylabel(' u(x,1)\rightarrow)'); end dt u1=firstupwind(t,dx,dt,a,x,u1,num); %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% function u1=firstupwind(t,dx,dt,a,x,u1,num)

c=a*dt/dx; if c>1 fprintf('c=%g, Enter another Courant Number\n',c); return else for i=1:int32(t/dt) for j=2:x-1 u1(j,i+1)=u1(j,i)-c*(u1(j,i)-u1(j-1,i)); end end end

filename='firstupwind.xlsx'; sheet= num; xlswrite(filename,u1(:,[1 t/(dt*6)+1 t/(dt*3)+1 t/(dt*2)+1 (2*t)/(dt*3)+1 (5*t)/(dt*6)+1 t/dt+1]),sheet,'B3'); figure surf(u1);