Missing "add reference" button and error CS1703

Hello, I need to add reference to "System.Drawing.dll" to my project so I can use Bitmap class.

But there is no "Add reference" button in "Project" drop-down menu or when I'm clicking RMB on project->add. I have only "add analyzer" and when I added "analyzer"like it was reference - nothing happened.

So I copied the .dll to Assets/Plugin, but I'm now getting error

 error CS1703: Multiple assemblies with equivalent identity have been imported: 'C:\Users\my\Desktop\project\project\Assets\Plugins\System.Drawing.dll' and 'C:\Program Files\Unity\Hub\Editor\2019.2.12f1\Editor\Data\NetStandard\compat\2.0.0\shims\netfx\System.Drawing.dll'. Remove one of the duplicate references.

I have no idea what to do.