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

Here is Why You Need An Adapter

The outlet and plug type used in Saint Lucia is G 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

Saint Lucia

In Saint Lucia, outlets of type G are in use.

Type G