I had a very similar problem while recently setting up a mesh with 2x RT-AC88U:
1. With latest Merlin 386.7_2 release on both, by any combination of wireless or wired connection, the prospective node unit would be recognised as available, but when confirmed to join, the settings screen would get to 1% then abort with the "unable to add node" popup.
2. With latest Merlin 386.7_2 release on mesh root and latest stock AsusWRT 386_48260 on node, the exact same result was seen.
3. Only when I downgraded the mesh root to Merlin 386.5_2 and left the node as it was, did everything function perfectly smoothly, quickly and wirelessly. [am still testing long-term reliability of this setup, with wireless 5GHz backhaul, but so far it is looking great]
4. My interim conclusion [open to correction, ofc] is that Merlin 386.7_2 is borked for mesh, at least on my model and the same may well apply to your closely related one.
Good luck!