Every time I try to load an anima file into UE 4.27 I get this error some of the assets required by the scene are not installed or they are locked and this plugin requires access to the original files
Were you able to figure this out? I just got this error as well:
Unable to export scene to unreal. I get this error:
Failed to open scenes
Reason: anima Plugin error: cannot load scene #0. Some of the assets required by the scene are not installed or they’re locked, and this plugin requires access to the original files.
Generic Male_Walk slant right is not installed.
Did you ever figured the reason… I have the same problem and this is the only thread I found 
I had the same problem when trying to import Anima Project to UE 4.27.
In Anima, when I added a " Talking group | 4 people" to the anima scene, there’s this one green shirted guy by default ( cman0204m4 ). Which has an invalid walking animation applied on (although it’s Idling in the group).
I ungrouped the 4 people with right click, selected the green shirted guy and changed his ‘Locomotion Walk Clip’ to something else.
I noticed that some of the other characters has invalid animations in some groups aswell. Just ungroup and change the animations.
Hope this helps.