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

Here is Why You Need An Adapter

The outlet and plug type used in Guinea-Bissau is C and in Eritrea are C and L. This is how they look like.

Eritrea

In Eritrea, outlets of type C and L are in use.

Type C

Type L

Guinea-Bissau

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

Type C