It's "webdrivers" not "webdriver" !

Gemfile

@@ -94,7 +94,7 @@ group :test do
   # Adds support for Capybara system testing and selenium driver
   gem 'capybara', '>= 2.15'
   gem 'selenium-webdriver'
-  gem 'webdriver'
+  gem 'webdrivers'
   gem 'launchy'
 
   gem "database_cleaner"

Gemfile.lock

@@ -341,7 +341,10 @@ GEM
       activemodel (>= 6.0.0)
       bindex (>= 0.4.0)
       railties (>= 6.0.0)
-    webdriver (0.3.0)
+    webdrivers (4.1.2)
+      nokogiri (~> 1.6)
+      rubyzip (~> 1.0)
+      selenium-webdriver (>= 3.0, < 4.0)
     webpacker (4.0.7)
       activesupport (>= 4.2)
       rack-proxy (>= 0.6.1)
@@ -403,7 +406,7 @@ DEPENDENCIES
   turbolinks (~> 5)
   tzinfo-data
   web-console (~> 4.0)
-  webdriver
+  webdrivers
   webpacker (~> 4.0)
   whenever