To answer your question, you'll get Marshal eventually, but not by scanning his amiibo. Marshal will move into your village eventually via an empty plot. I've had this happen before, it's known as the villager voice mail glitch. It's a weird thing that happens that Nintendo never fixed. When you try to use an amiibo you can't access them and you get some variation on a message that they're not available. It's happening because Marshal is already in your void and is on the list to move in soon when an open plot comes up. Since he's already on the void list to move in, he won't come when you try to scan his amiibo for the third time.
In my case I realized what had happened was that I'd scanned my Apple amiibo twice and then visited another person's village she was moving out of. I didn't even talk to her, but she was still placed in my void and I got a similar message to what you did when I tried to scan her the third time. No matter how many times I scanned her amiibo I couldn't get her to come to the campsite. I had to wait till she eventually moved in naturally by taking an empty plot in my village. It was at that point she mentioned the name of my friend's town and I was able to confirm where and when she'd gone into my void. I don't time travel so it wasn't till two months after she entered my void that she finally chose to move in on her own and grace me with her presence. So Marshal should move in naturally eventually. I'm not sure if or how time travel would affect this. Once Marshal moves in things should go back to normal and if say Marshal came in and then you later had him move out again his Amiibo should work normally again. It isn't the Amiibo card itself that's malfunctioning.
