Hi i'm following installation instructions but actually i'm getting this error when i put 1.4.1 version in addons settings and try to activate plugin. 
The Godot version is 4.4.1, i've made C# solutions and using the Mono version of the engine.

Doh ! I had the same issue but its fixed by following the build instructions.

Don't forget to actually press the C# build button (hammer icon) in the top right run bar of the editor! 

~CookieBadger

Hi and thanks for the answers! Yes i think that i've forgot the build button, but when i press it i've another error:

Please closely follow the instructions: https://cookiebadger.github.io/assetplacer-docs/installation.html

You need to have .NET installed on your machine for C# scripts to work, the latest version can be downloaded here: https://dotnet.microsoft.com/en-us/download/dotnet