in one of written exercises I wrote the first line below and it’s then been corrected to read as the second line.
So, I’ve looked up the answer for reference and ‘in deren’ seems to be Genitive Plural and translates as ‘in which’. That would make sense in English too. However, in English it would be very normal to use ‘where’. and that is why I wrote wo. Is that not allowed in German?
Manchester, London und Edinburgh sind größe Stadte wo man viele spannende Aktivitäten machen kann.
Manchester, London und Edinburgh sind größe Stadte, in denen man viele spannende Aktivitäten machen kann.