* sequel's crystal ball is broken :-) It cannot detect that you want 'database' to be an sqlite-DB. Give it a litte help and change 'database' to 'sqlite:database' and you are good to go:
```
irb(main):001:0> require 'sequel' => true
irb(main):002:0> x=Sequel.connect('blah')
Traceback (most recent call last):
7: from /usr/bin/irb:23:in `<main>' 6: from /usr/bin/irb:23:in `load'
5: from /usr/lib64/ruby/gems/2.7.0/gems/irb-1.2.6/exe/irb:11:in `<top (required)>' 4: from (irb):2 3: from /usr/lib64/ruby/gems/2.7.0/gems/sequel-5.40.0/lib/sequel/core.rb:124:in `connect'
2: from /usr/lib64/ruby/gems/2.7.0/gems/sequel-5.40.0/lib/sequel/database/connecting.rb:36:in `connect' 1: from /usr/lib64/ruby/gems/2.7.0/gems/sequel-5.40.0/lib/sequel/database/connecting.rb:17:in `adapter_class'
NoMethodError (undefined method `to_sym' for nil:NilClass)
irb(main):003:0> x=Sequel.connect('sqlite:blah')
> => #<Sequel::SQLite::Database: "sqlite:blah">
```