Error detected while processing

[Yuriy]

I installed vim plugin according to README manual:

git clone https://github.com/soramugi/auto-ctags.vim.git ~/.vim/plugin/auto-ctags.vim.git

But when I open vim editor some errors were appeared:

Error detected while processing /home/username/.vim/plugin/auto-ctags.vim.git/autoload/auto_ctags.vim:
line   14:
E117: Unknown function: vital#autoctags#import
E15: Invalid expression: vital#autoctags#import('System.File')
line   15:
E117: Unknown function: vital#autoctags#import
E15: Invalid expression: vital#autoctags#import('System.Process')
line   16:
E117: Unknown function: vital#autoctags#import
E15: Invalid expression: vital#autoctags#import('System.Filepath')
line   17:
E117: Unknown function: vital#autoctags#import
E15: Invalid expression: vital#autoctags#import('System.Job')
line   18:
E117: Unknown function: vital#autoctags#import
E15: Invalid expression: vital#autoctags#import('Async.Promise')
Press ENTER or type command to continue

Vim version

VIM - Vi IMproved 8.0 

I did something wrong or it's bug? Thank You!