Is there a specific reason you're installing manually?
As opposed to the automated way:
https://stardewvalleywiki.com/Modding:Player_Guide/Getting_Started#Install_SMAPI
If you can't find the file.
You may want to check your OS for any settings to show 'file extensions'. As your OS may be hiding the extension from you.
I'm doing it manually because when I do it automatically it still says it's missing that file. When I went into my OS and I couldn't find anything and changed settings and still nothing showed up.
Here is a copy of the msg that shows up when I do it automatically
{(That's all I need! I'll install SMAPI now.
Game path: C:\Program Files (x86)\Steam\steamapps\common\Stardew Valley
Color scheme: auto-detect
----------------------------------------------------------------------------
Removing previous SMAPI files...
Adding SMAPI files...
The installer failed with an unexpected exception.
If you need help fixing this error, see
https://smapi.io/help
System.IO.FileNotFoundException: Could not find file 'C:\Program Files (x86)\Steam\steamapps\common\Stardew Valley\Stardew Valley.deps.json'.
File name: 'C:\Program Files (x86)\Steam\steamapps\common\Stardew Valley\Stardew Valley.deps.json'
at System.IO.FileSystem.CopyFile(String , String , Boolean )
at System.IO.File.Copy(String , String , Boolean )
at StardewModdingApi.Installer.InteractiveInstaller.Run(String[] args) in E:\source\_Stardew\SMAPI\src\SMAPI.Installer\InteractiveInstaller.cs:line 421
at StardewModdingApi.Installer.Program.Main(String[] args) in E:\source\_Stardew\SMAPI\src\SMAPI.Installer\Program.cs:line 55
Game has ended. Press any key to exit.})