You Do Need power plug adapter while traveling from Serbia to Tuvalu.

Here is Why You Need An Adapter

The outlet and plug type used in Tuvalu is I and in Serbia are C and F. This is how they look like.

Serbia

In Serbia, outlets of type C and F are in use.

Type C

Type F

Tuvalu

In Tuvalu, outlets of type I are in use.

Type I