The project from Keijiro maybe lose access to RtMidi.dll that you can find here:

https://github.com/keijiro/jp.keijiro.rtmidi/tree/master/Packages/jp.keijiro.rtmidi/Runtime/Windows

Inside this scrpit for instance, you have this import: using RtMidiDll = RtMidi.Unmanaged;

https://github.com/keijiro/Minis/blob/master/Packages/jp.keijiro.minis/Runtime/Internal/MidiPort.cs

To solve it you can import manuality from Keijiro repository.