Autoit - "possibly used before declaration" and "undeclared global variable" Compile Error - autoit

Question

This is my first aproach to AutoIt and my apologies for the basic questions
I have an AutoIt program/code that is made of 2 .au3 files
1 - I would like to ask, how does that files work together (how they can be combined, if this is possbile) or are they supposed to be 2 seperate programs?
2 - When I run AutoIt3_x64, in one of them, I get the error
Line xxxx
Error: Variable used without being declared.
I have run Au3Stripper before compile and I got the followin errors/warning
-> 3 Change(s) made.
>Running AU3Check (3.3.14.5) from:C:\..\AutoIt3 input:C:\..\project.au3
"C:\..\project.au3"(203,110) : warning: $key: possibly used before declaration.
File($CommonFilesDir & "\" & $first_run, $CommonFilesDir & "\" & $first_run & $first_run, $key,
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
"C:\..\project.au3"(203,110) : error: $key: undeclared global variable.
File($CommonFilesDir & "\" & $first_run, $CommonFilesDir & "\" & $first_run & $first_run, $key,
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
"C:\..\project.au3"(273,32) : error: server(): undefined function.
$get_table = server()
~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
C:\..\project.au3 - 2 error(s), 1 warning(s)
!>06:52:51 AU3Check ended. Press F4 to jump to next error.rc:2
+>06:52:51 AutoIt3Wrapper Finished.
What is causing these errors/warning and how can I fix it?
Thank you
EDIT:
Unfortunately, following the link sugestions, I didn't managed to succeed and for sure it's my fault. Th structure of the link exemple is diferente
one of the errors refers to this part of the code
if not FileExists($CommonFilesDir & "\" & $first_run & $first_run) Then
File($CommonFilesDir & "\" & $first_run, $CommonFilesDir & "\" & $first_run & $first_run, $key)
FileWrite($CommonFilesDir & "\" & $reverse_diskid, Get_Table())
FileDelete($CommonFilesDir & "\" & $first_run)
endif
I don't see how can I replicate the link suggestion