format PE console 4.0

include 'win32a.inc'

entry start

section '.data' data readable
array db "1a2b3c4d",0
fmt db "%c",0

section '.code' readable executable
start:
   mov ecx, 1
  
mainLoop:
   cmp ecx, 7
  jne print 
  je okay
     
print: 
     inc ecx
     cinvoke printf, fmt, [array+eax]
    jmp mainLoop
        
okay:
       invoke ExitProcess, 0

section '.idata' data import readable writeable
library kernel32, 'kernel32.dll',  msvcrt, 'msvcrt.dll'
import kernel32, ExitProcess, 'ExitProcess'
import msvcrt, msvcrt, 'printf'
    

flat assembler version 1.70 (1701741 kilobytes memory)
arraytest.asm [22]:
cinvoke printf, fmt, [array+eax]
C:\FASM\INCLUDE/macro/proc32.inc [41] cinvoke [4]:
pushd arg
error: invalid size of operand.

format PE console 4.0 

include 'win32a.inc' 

entry start 

section '.data' data readable 
array     db "1a2b3c4d",0 
fmt       db "%c",0 

section '.code' readable executable 
start: 
        mov        ecx, 0 
         
mainLoop: 
        cmp   ecx, 8
        jne   print  
        je   okay 
         
print: 
        lea        eax, [ecx + array]
        push      ecx
        cinvoke  printf, fmt, [eax]
        pop       ecx
        
        inc  ecx
        jmp      mainLoop 
         
okay: 
        invoke  ExitProcess, 0 

section '.idata' data import readable writeable 
library kernel32, 'kernel32.dll',  msvcrt, 'msvcrt.dll' 
import kernel32, ExitProcess, 'ExitProcess' 
import msvcrt, printf, 'printf'     

But why push ecx and pop ecx are also needed?

In WinApi; esi, edi and ebx are the only register preserved. Other may and certenly will be modified by the function called in the api.

in your case, is better to use one of the preserved registers, so you don´t need to make the push pop.