Cannot run Fortran code - fortran

Question

My program runs with errors and I really have no idea why, will you help me please? The question is as below:
The flow problem is classic example of viscous diffusion. The governing equation for such problem was derived using boundary layer theory to reduce the full Navier- Stokes equations to the single parabolic PDE.
with the necessary initial and boundary conditions,
t = 0: u(0) = 0, u(0.04m) = 0;
t > 0: u(0) = 40.0, u(0.04m) = U = 0.0m/s.
This problem may be described physically as transient viscous-driven flow between two plates of infinite extent and separated by a distance of 0.04m. Initially both plates are at rest. After time,t=0, the upper plate is set in motion in the positive x-direction with a velocity of 40.0m/s. Due to the viscosity of the fluid filling the space between the plates, successive lamina of fluid are set in motion as time elapses. Eventually, the system reaches a “quasi-steady state”, as the velocity profile becomes more or less constant in time. The governing equation lends nicely to the use of finite difference techniques to solve the problem in the transient domain.
Computer Code in Fortran for DUFORT FRANKEL SCHEME.
! Homework1 DUFORT FRANKE SCHEME
! Program computes the numerical solution to the
! Transient Flow Problem.
! The following initial and bounadry conditions are applied:
! t=0: u(y=0)=40.0m/s
! t>0: u(y=0)=0.0; u(y=0.04m)=0.0
parameter(maxn=30,eps=1.0e-3)
integer k,m,mm,count
real*8 u_old(1001,maxn),u_new(1001,maxn),y(maxn)
real*8 t,tau,h,r,tmax,u_init,nu,sum,error
!
data h,m,u_init,nu,r,tmax /0.001,41,0.0,2.17e-4,0.217,2.5e5/
!
open(unit=1,file='hw1_dufort.out',status='unknown')
tau=r*h**2/nu
mm=m-1
error=1.0
!
count=0
k=1
t=0.0
y(1)=0.0
!
do 2 i=2,m
y(i)=y(i-1)+h
2 continue
!
write(1,*)'Velocity Results:'
write(1,10)t,(u_old(k,j),j=1,m)
do while ((error.gt.eps).and.(count.lt.1080))
count=count+1
sum=0.0
t=t+tau
u_old(k,1)=40.0
u_old(k,m)=0.0
do 4 i=2,mm
if (k.lt.2) then
u_new(k,i)=(2.0*r/(1.0+2.0*r))*(u_old(k,i+1)+u_old(k,i-1))
else
u_new(k,i)=(2.0*r/(1.0+2.0*r))*(u_old(k,i+1)+u_old(k,i-1))+ ((1.0 -
& 2.0*r)/(1.0+2.0*r))*u_old(k-1,i)
end if
end do
!
10 format(2x,f10.3,2x,41f8.4)
write(1,'(" Number of steps for convergence = ",i4)')count
end