The symbol SHUTDOWN is the name of a routine whose interface is declared in WIN32API.INS. If you include this file in your source code, or use a module that contains an interface to the routine, you cannot use SHUTDOWN as the name of a common block in the same program unit.
Here is an example code that gives the same compile time error message as in the original post.
program tsh
implicit none
include 'win32api.ins'
integer ixy
common /shutdown/ixy
!
ixy = 3
print *,ixy
end program
Attempting to compile this gives
s:\FTN95>ftn95 sd.f90 /check
[FTN95/Win32 Ver. 8.30.279 Copyright (c) Silverfrost Ltd 1993-2018]
0005) common /shutdown/ixy
*** COMMON block /SHUTDOWN/ is already in use as another global entity
1 ERROR [<TSH> FTN95 v8.30.279]
*** Compilation failed