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

Here is Why You Need An Adapter

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

Guinea-Bissau

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

Type C

Namibia

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

Type D

Type M