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

Here is Why You Need An Adapter

The outlet and plug type used in Guinea-Bissau is C and in Namibia are D and M. This is how they look like.

Namibia

In Namibia, outlets of type D and M are in use.

Type D

Type M

Guinea-Bissau

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

Type C