Ype Mismatch Error When Assigning Array[Enemy] in GDScript

Sounds like you’ve hit this issue Less strict design for typed arrays · Issue #73005 · godotengine/godot · GitHub

You should be able to use assign to work around the issue.

var enemy_nodes: Array[Node] = get_tree().get_nodes_in_group(str(Groups.Values.ENEMIES))
var enemies: Array[Enemy] = []
enemies.assign(enemy_nodes)