It’s just that the word förra requires a definite form. The word nästa (next) does not however. Sorry.
It doesn't mean the same thing. You can say den sista veckan, but that is the last week (as in 'the final week').
When you say 'last week' as a standalone phrase in English with no context, it will be understood as 'the week before this one'.
i have difficulties with this one as it displays "last week" as a correct answer on a similar exercise and incorrect with "previous week"?