You Do Need power plug adapter while traveling from Nauru to Guinea-Bissau.

Here is Why You Need An Adapter

The outlet and plug type used in Guinea-Bissau is C and in Nauru is I. This is how they look like.

Nauru

In Nauru, outlets of type I are in use.

Type I

Guinea-Bissau

In Guinea-Bissau, outlets of type C are in use.

Type C