System.Threading.Tasks.Extensions.dll is not allowed to be included or could not be found

Hello, I have this game that uses npgsql.dll for database and I'm trying to build the game and the following error shows up:

ArgumentException: The Assembly System.Threading.Tasks.Extensions is referenced by Npgsql ('Assets/plugin/Npgsql.dll'). But the dll is not allowed to be included or could not be found. UnityEditor.AssemblyHelper.AddReferencedAssembliesRecurse (System.String assemblyPath, System.Collections.Generic.List1[T] alreadyFoundAssemblies, System.String[] allAssemblyPaths, System.String[] foldersToSearch, System.Collections.Generic.Dictionary2[TKey,TValue] cache, UnityEditor.BuildTarget target) (at C:/buildslave/unity/build/Editor/Mono/AssemblyHelper.cs:150) UnityEditor.AssemblyHelper.FindAssembliesReferencedBy (System.String[] paths, System.String[] foldersToSearch, UnityEditor.BuildTarget target) (at C:/buildslave/unity/build/Editor/Mono/AssemblyHelper.cs:190) UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)

I have both dll(Npgsql.dll and System.Threading.Tasks.Extensions.dll) in Assets\Plugins