I've just run into this exact same situation. This is related to the new villager moving void/cache bug. If you've never heard of it before you can read up on it
here.
There is a fix as long as you have multiple amiibo cards.
What caused my problem:
Day 1: I invited Marshal to my campsite via amiibo card, did one DYI for him
Day 2 - part 1: Sterling asks to move out and I tell him to go
Day 2 - part 2: I visit my friends island. My friend recently had Marshal move out from her island a few days ago. On the day of my visit, she had Marshal re-invited to her campsite via amiibo card. I never talked to Marshal at her island's campsite.
Day 2 - part 3: I return to my island and try to invite Marshal to my campsite via amiibo. I get the message saying "
he's in the middle of moving and can't visit me". At this point I'm worried he's somehow glitched, and moving into Sterling's plot, despite Sterling still being in the village and not in cardboard boxes yet.
Day 3: I still can't invite Marshal to my campsite via amiibo, says he's moving. Sterling is in cardboard boxes moving out.
Day 4
(and might be part of the solution for you): Sterling is gone, plot is empty and for sale. I decide to
let the empty plot fill up naturally by moving forward a day rather than recruiting a villager from a Nook Mile Island in case Marshal somehow take over the plot.
Day 5: Marshal's name is on the sold sign of Sterling's old plot
Day 6: Marshal's house is built but when I click on his house door it says "
I'm moving". At this point I know my game is glitched as it's the exact scenario of the void glitch I linked above. I see Marshal on my town map, but he's no where to be found on the island.
Fixing the glitched moved in villager:
Invite another amiibo villager and boot out the glitched "I'm moving out" villager. I invited Mitzi, did 3 days of DYI, then booted the glitched Marshal.
After Mitzi moved in I was able to re-invite Marshal via amiibo. I spent another 3 days doing DYI for Marshal, then he moved in just fine and not glitched.
Hope this helps, good luck.
