let me explain the error first, before making suggestions to fix it
when you use this construction...
[WHERE] [I]expression[/I] IN ( [I]subquery[/I] )
you must be sure that the subquery returns only one column, so that the expression, which is a single value, can be compared to these individual column values, looking for a match
your subquery returns two columns
try this instead --
WHERE region = 'europe'
AND gdp/population =
( SELECT gdp/population
WHERE name = 'united kingdom' )
note the use of AND in the WHERE clause, to combine two conditions
you should also learn to use formatting in your code, with line breaks and indentation -- you will thank me later