Using global variables for kata state is Bad Design(TM). Please use a proper class to encapsulate the entire operation.

with a 2-Down-1-Up procedure, one needs to give the correct response twice before the task can become harder

It's not clear how subsequent correct responses interact with 2D1U. Does it take 2 more responses before the task become harder again? Or does it go harder again each response.

Also, sometimes description writes D12U when it should be 2D1U.

Ruby 3.0 should be enabled, see this to learn how to do it