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

Here is Why You Need An Adapter

The outlet and plug type used in Guinea are C, F and K 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

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

Type C

Type F

Type K