You need a paid membership to Nintendo Switch Online to play

Fortnite for Switch won't require Nintendo's premium online service for play - Polygon

How Much Does Nintendo Online Cost?

Level up with a Nintendo Switch Online Membership - Nintendo Official Site

Nintendo Switch Online - Nintendo - Official Site

Level up with a Nintendo Switch Online Membership - Nintendo Official Site

How To FIX A paid membership to Nintendo Switch Online is required on Nintendo Switch!

Nintendo Switch Online + Expansion Pack 12-month Individual Membership – [Digital Code] : Everything Else

You'll have to pay to play Nintendo Switch games online — except for 'Fortnite' (NTDOY)

Nintendo Switch Online - Nintendo - Official Site

Nintendo Switch Online - Nintendo - Official Site