Dynamics Ax 2009 - The selected file is not a valid .NET assembly

Hi,
when I try to add the dll as a reference in Dynamics Ax 2009 it says "The selected file is not a valid .NET assembly, therefore the reference cannot be added.". What am I missing here?

File is WinSCPnet.dll, I've tried them all: the ones in the net40 and netstandard2.0 directory and also the one in the main directory WinSCP-6.1.2-Automation. I've downloaded the said files from here: https://winscp.net/download/WinSCP-6.1.2-Automation.zip/download

Thanks in advance

I do not know Dynamics Ax. But based on "2009": Isn't it using .NET framework version older than 4.0?