This server will operate in LAN mode only

If the server displays this error in server console.

Initializing Steam libraries for secure Internet server
*********************************************************
*********************************************************
* Unable to load Steam support library.*
* Unable to load Steam support library.*
* This server will operate in LAN mode only.*
* This server will operate in LAN mode only.*
*********************************************************
*********************************************************

check the root folder of the server and look for these files

Mode LastWriteTime Length Name

---

-a---- 9/1/2021 5:42 PM 17510120 steamclient.dll
-a---- 9/1/2021 5:42 PM 20987856 steamclient64.dll
-a---- 9/1/2021 5:42 PM 350440 tier0_s.dll
-a---- 9/1/2021 5:42 PM 431336 tier0_s64.dll
-a---- 9/1/2021 5:42 PM 320744 vstdlib_s.dll
-a---- 9/1/2021 5:42 PM 479976 vstdlib_s64.dll

if you don't see these files, can try to copy them from another source or steam server.