[DEPRECATION] requiring "RMagick" is deprecated. Use "rmagick" instead

[JeffFan38]

Hi,

When I install the Knowledgebase plugin on my Redmine system, it shows below message :

[root@localhost plugins]# rake redmine:plugins:migrate NAME=redmine_knowledgebase
(in /var/www/redmine)
[DEPRECATION] requiring "RMagick" is deprecated. Use "rmagick" instead
rake aborted!
LoadError: cannot load such file -- ap
/var/www/redmine/config/application.rb:5:in <top (required)>' /var/www/redmine/Rakefile:5:in <top (required)>'
(See full trace by running task with --trace)

My system's config :
Environment:
Redmine version 4.0.3.stable
Ruby version 2.6.3-p62 (2019-04-16) [x86_64-linux]
Rails version 5.2.2.1
Environment production
Database adapter Mysql2

I am not familiar on the Rail, Rake. Can someone help to check how to install the Knowledgebase plugin on my Redmine system?
Thanks.