Is Reaper VST3 compatible?

This is necessary to use the VST Editor within the DAW. Your Program is reporting that you have not selected the ports properly.

Port 1 should be selected where it indicates Port 1
Port 4 should be selected where it indicates Port 4

Port 1 is used for direct control over the hardware synthesizer, the MOXF.
Port 4 is used exclusively to control the Editor, the MOXF6/MOXF8 EDITOR VST.

Sorry, I’m not familiar with Reaper, is it supposed to support “VST3” Protocol. If yes, you can run it inside of your DAW. If not, you can run it as a stand-alone application alongside your DAW.

Contact the makers of Reaper, you need to be able to have Ports 1 and 4 active.
PORT 1 for MIDI Track Data.
PORT 4 for communication with the Editor.

You need to be able to change that Port 1 “Not Assigned” for MIDI OUT to MOXF Port 1
You need to be able to select “MOXF6/MOXF8 Port 4” for both MIDI IN and OUT where it is asking for Port 4.